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.

ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws