provider/aws: Don't set DBName on `aws_db_instance` from snapshot
provider/aws: Don't set DBName on `aws_db_instance` from snapshot

It turns out if you're trying to write a config to conditionally restore
an instance from a snapshot, you end up painted into a corner with the
combination of `snapshot_identifier` and `name`.

You need `name` to be specified for DBs you're creating, but when
`snapshot_identifier` is populated you need it to be blank or else the
AWS API throws an error.

The logical next step is to drop a ternary in:

```tf
resource "aws_db_instance" "foo" {
 # ...
 snapshot_identifier = "${var.snap}"
 name = "${var.snap != "" ? "" : var.dbname}"
}
```

**BUT** the above config will _replace_ the DB on subsequent runs as the
config basically has `name = ""` which will trigger a ForceNew diff once
the `name` is populated from the snapshot restore.

**SO** we can get a workable solution by actively avoiding populating
DBName when we're using one of the engines the docs explicitly mention.

It does not look like there are any tests covering `snapshot_identifer`,
so I'll subject this to some manual tests and follow up with some
results.

pdecat/terraform,youhong316/terraform,antonbabenko/terraform,hartzell/terraform,iansheridan/terraform,golvteppe/terraform,evalphobia/saas-ci-performance-compare-terraform,KensoDev/terraform,youhong316/terraform,GannettDigital/terraform,mathieuherbert/terraform,ewypych/terraform,gazoakley/terraform,dougneal/terraform,Ticketmaster/terraform,hooklift/terraform,dgolja/terraform,svanharmelen/terraform,dwradcliffe/terraform,pgray/terraform,pdecat/terraform,bashtoni/terraform,cloudnautique/terraform,evandbrown/terraform,samdunne/terraform,cnicolov/terraform,ReleaseQueue/terraform,iansheridan/terraform,paulbouwer/terraform,heimweh/terraform,ctavan/terraform,whiskeyjay/terraform,kristinn/terraform,ddegoede/terraform,brandontosch/terraform,kyhavlov/terraform,jrperritt/terraform,dwradcliffe/terraform,jcaugust/terraform,GrayCoder/terraform,mathieuherbert/terraform,richardc/terraform,mbfrahry/terraform,luis-silva/terraform,TheWeatherCompany/terraform,lusis/terraform,mbfrahry/terraform,mtougeron/terraform,nathanielks/terraform,10thmagnitude/terraform,caiofilipini/terraform,ryon/terraform,VladRassokhin/terraform,displague/terraform,tpoindessous/terraform,ericwestfall/terraform,ross/terraform,kristinn/terraform,randomvariable/terraform,paybyphone/terraform,PaulAtkins/terraform,glerchundi/terraform,cmorent/terraform,nathanielks/terraform,mattray/terraform,ColinHebert/terraform,hrach/hashicorp-terraform,niclic/terraform,caiofilipini/terraform,Wambosa/terraform,HotelsDotCom/terraform,rhyas/terraform,justnom/terraform,bashtoni/terraform,dgolja/terraform,antonbabenko/terraform,glerchundi/terraform,cmorent/terraform,billyschmidt/terraform,displague/terraform,KensoDev/terraform,huydinhle/terraform,fatih/terraform,DevoKun/terraform,kyhavlov/terraform,justnom/terraform,taliesins/terraform,mathematician/terraform,glerchundi/terraform,rrudduck/terraform,jrperritt/terraform,Ticketmaster/terraform,whiskeyjay/terraform,tpoindessous/terraform,jhedev/terraform,kyhavlov/terraform,dwradcliffe/terraform,gazoakley/terraform,harijayms/terraform,julia-stripe/terraform,chrisjharding/terraform,drebes/terraform,ddegoede/terraform,mevansam/terraform,ryon/terraform,fatih/terraform,kristinn/terraform,VladRassokhin/terraform,hartzell/terraform,gechr/terraform,jhedev/terraform,golvteppe/terraform,chrisjharding/terraform,dn1s/terraform,richardc/terraform,ddegoede/terraform,niclic/terraform,mafrosis/terraform,pgray/terraform,fatih/terraform,VladRassokhin/terraform,hartzell/terraform,dn1s/terraform,brunomcustodio/terraform,mtougeron/terraform,evalphobia/saas-ci-performance-compare-terraform,taliesins/terraform,glerchundi/terraform,chrisjharding/terraform,catsby/terraform,brianantonelli/terraform,ewypych/terraform,lifesum/terraform,tmshn/terraform,hashicorp/terraform,GrayCoder/terraform,ryon/terraform,ross/terraform,DevoKun/terraform,rhyas/terraform,appbricks/ab-terraform-build,ericwestfall/terraform,vancluever/terraform,StephenWeatherford/terraform,asedge/terraform,schans/terraform,catsby/terraform,kristinn/terraform,nathanielks/terraform,ctavan/terraform,wowgroup/terraform,mattray/terraform,pryorda/terraform,DevoKun/terraform,sl1pm4t/terraform,StephenWeatherford/terraform,matt-deboer/terraform,julia-stripe/terraform,nlamirault/terraform,jhedev/terraform,iljaskevic/terraform,mathematician/terraform,catsby/terraform,hrach/hashicorp-terraform,asedge/terraform,Ticketmaster/terraform,justnom/terraform,donaldguy/terraform,JDiPierro/terraform,kl4w/terraform,PeteGoo/terraform,koding/terraform,wowgroup/terraform,hooklift/terraform,paulbouwer/terraform,TheWeatherCompany/terraform,hrach/hashicorp-terraform,paybyphone/terraform,mtougeron/terraform,schans/terraform,mathematician/terraform,maxenglander/terraform,mattray/terraform,pradeepbhadani/terraform,ryon/terraform,lstolyarov/terraform,bashtoni/terraform,kwilczynski/terraform,wowgroup/terraform,kjmkznr/terraform,donaldguy/terraform,ewbankkit/terraform,cloudnautique/terraform,grasshoppergroup/terraform,appbricks/ab-terraform-build,gazoakley/terraform,matt-deboer/terraform,caiofilipini/terraform,rhyas/terraform,tpoindessous/terraform,billyschmidt/terraform,whiskeyjay/terraform,sl1pm4t/terraform,gechr/terraform,schans/terraform,samdunne/terraform,golvteppe/terraform,ColinHebert/terraform,fatmcgav/terraform,lstolyarov/terraform,dgolja/terraform,golvteppe/terraform,lifesum/terraform,appbricks/ab-terraform-build,rkhozinov/terraform,tombuildsstuff/terraform,hashicorp/terraform,rhyas/terraform,rnaveiras/terraform,paultyng/terraform,huydinhle/terraform,jhedev/terraform,brandontosch/terraform,mafrosis/terraform,mathieuherbert/terraform,rkhozinov/terraform,tommynsong/terraform,niclic/terraform,HotelsDotCom/terraform,ross/terraform,mevansam/terraform,iljaskevic/terraform,yanndegat/terraform,Wambosa/terraform,randomvariable/terraform,grasshoppergroup/terraform,asedge/terraform,rrudduck/terraform,jcaugust/terraform,fatih/terraform,kwilczynski/terraform,brandontosch/terraform,elblivion/terraform,elblivion/terraform,schans/terraform,dwradcliffe/terraform,markpeek/terraform,drebes/terraform,JDiPierro/terraform,cloudnautique/terraform,lstolyarov/terraform,Boran/terraform,StephenWeatherford/terraform,youhong316/terraform,paybyphone/terraform,evandbrown/terraform,ctavan/terraform,lifesum/terraform,nathanielks/terraform,yanndegat/terraform,richardc/terraform,nathanielks/terraform,dgolja/terraform,tpounds/terraform,ReleaseQueue/terraform,matt-deboer/terraform,jrperritt/terraform,pryorda/terraform,travertischio/terraform,StephenWeatherford/terraform,svanharmelen/terraform,drebes/terraform,ewbankkit/terraform,KensoDev/terraform,lstolyarov/terraform,lifesum/terraform,antonbabenko/terraform,Boran/terraform,koding/terraform,rnaveiras/terraform,hashicorp/terraform,mtougeron/terraform,markpeek/terraform,StephenWeatherford/terraform,GannettDigital/terraform,paulbouwer/terraform,golvteppe/terraform,harijayms/terraform,mevansam/terraform,tmshn/terraform,tmshn/terraform,iansheridan/terraform,mtougeron/terraform,evandbrown/terraform,billyschmidt/terraform,rrudduck/terraform,hrach/hashicorp-terraform,heimweh/terraform,vancluever/terraform,ericwestfall/terraform,heimweh/terraform,wowgroup/terraform,bradfeehan/terraform,maxenglander/terraform,10thmagnitude/terraform,PeteGoo/terraform,paultyng/terraform,asedge/terraform,cloudnautique/terraform,VladRassokhin/terraform,PaulAtkins/terraform,koding/terraform,samdunne/terraform,KensoDev/terraform,heimweh/terraform,brandontosch/terraform,julia-stripe/terraform,vancluever/terraform,paultyng/terraform,evalphobia/saas-ci-performance-compare-terraform,justnom/terraform,JDiPierro/terraform,HotelsDotCom/terraform,maxenglander/terraform,lstolyarov/terraform,tpoindessous/terraform,gazoakley/terraform,PaulAtkins/terraform,youhong316/terraform,paulbouwer/terraform,bradfeehan/terraform,tombuildsstuff/terraform,pgray/terraform,nlamirault/terraform,evandbrown/terraform,ReleaseQueue/terraform,fatmcgav/terraform,ddegoede/terraform,JDiPierro/terraform,lusis/terraform,apparentlymart/terraform,grasshoppergroup/terraform,koding/terraform,luis-silva/terraform,rnaveiras/terraform,cmorent/terraform,kjmkznr/terraform,dougneal/terraform,chrisjharding/terraform,Boran/terraform,DevoKun/terraform,bashtoni/terraform,paulbouwer/terraform,mbfrahry/terraform,brunomcustodio/terraform,kjmkznr/terraform,ericwestfall/terraform,tpounds/terraform,kl4w/terraform,jrperritt/terraform,10thmagnitude/terraform,fatmcgav/terraform,brianantonelli/terraform,10thmagnitude/terraform,ewbankkit/terraform,svanharmelen/terraform,Pryz/terraform,cmorent/terraform,mbfrahry/terraform,randomvariable/terraform,hrach/hashicorp-terraform,apparentlymart/terraform,cmorent/terraform,Pryz/terraform,julia-stripe/terraform,mathieuherbert/terraform,Pryz/terraform,ewypych/terraform,markpeek/terraform,chrisjharding/terraform,matt-deboer/terraform,gechr/terraform,GannettDigital/terraform,displague/terraform,KensoDev/terraform,tpounds/terraform,sl1pm4t/terraform,tommynsong/terraform,tommynsong/terraform,PeteGoo/terraform,GannettDigital/terraform,ReleaseQueue/terraform,grasshoppergroup/terraform,dougneal/terraform,heimweh/terraform,HotelsDotCom/terraform,tmshn/terraform,evalphobia/saas-ci-performance-compare-terraform,yanndegat/terraform,ColinHebert/terraform,HotelsDotCom/terraform,harijayms/terraform,travertischio/terraform,lifesum/terraform,mathematician/terraform,kl4w/terraform,Wambosa/terraform,richardc/terraform,brianantonelli/terraform,paybyphone/terraform,iljaskevic/terraform,yanndegat/terraform,cnicolov/terraform,nlamirault/terraform,fatih/terraform,pradeepbhadani/terraform,dougneal/terraform,koding/terraform,tpounds/terraform,cloudnautique/terraform,rrudduck/terraform,vancluever/terraform,ColinHebert/terraform,whiskeyjay/terraform,kwilczynski/terraform,apparentlymart/terraform,travertischio/terraform,appbricks/ab-terraform-build,catsby/terraform,kjmkznr/terraform,dougneal/terraform,taliesins/terraform,mathieuherbert/terraform,billyschmidt/terraform,randomvariable/terraform,randomvariable/terraform,10thmagnitude/terraform,iljaskevic/terraform,donaldguy/terraform,TheWeatherCompany/terraform,jcaugust/terraform,julia-stripe/terraform,brandontosch/terraform,Ticketmaster/terraform,billyschmidt/terraform,markpeek/terraform,displague/terraform,tombuildsstuff/terraform,ddegoede/terraform,Pryz/terraform,mafrosis/terraform,brianantonelli/terraform,maxenglander/terraform,mafrosis/terraform,maxenglander/terraform,evalphobia/saas-ci-performance-compare-terraform,pradeepbhadani/terraform,drebes/terraform,JDiPierro/terraform,niclic/terraform,iansheridan/terraform,jcaugust/terraform,brunomcustodio/terraform,GrayCoder/terraform,svanharmelen/terraform,PeteGoo/terraform,PaulAtkins/terraform,paultyng/terraform,pryorda/terraform,ctavan/terraform,GrayCoder/terraform,rkhozinov/terraform,bradfeehan/terraform,pdecat/terraform,harijayms/terraform,elblivion/terraform,huydinhle/terraform,pgray/terraform,TheWeatherCompany/terraform,elblivion/terraform,displague/terraform,brianantonelli/terraform,tpoin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