Add `post_downgrade` hook for migration tests.
Add `post_downgrade` hook for migration tests.

This can be used by tests to assert the state of the database after a
`downgrade` has been run.

This patch also renames `_prerun_#` to `_pre_upgrade` to match.

Change-Id: Id2a100a8270835d1ef6f9908ff5b89e2b531bb3c

devendermishrajio/nova,luogangyi/bcec-nova,cloudbase/nova,sebrandon1/nova,vmturbo/nova,j-carpentier/nova,mandeepdhami/nova,gspilio/nova,mikalstill/nova,CCI-MOC/nova,double12gzh/nova,affo/nova,akash1808/nova,cloudbau/nova,eonpatapon/nova,redhat-openstack/nova,badock/nova,noironetworks/nova,ruslanloman/nova,MountainWei/nova,tanglei528/nova,sacharya/nova,SUSE-Cloud/nova,orbitfp7/nova,belmiromoreira/nova,tanglei528/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,klmitch/nova,Triv90/Nova,cernops/nova,ewindisch/nova,varunarya10/nova_test_latest,klmitch/nova,rajalokan/nova,belmiromoreira/nova,spring-week-topos/nova-week,openstack/nova,badock/nova,BeyondTheClouds/nova,fnordahl/nova,yatinkumbhare/openstack-nova,berrange/nova,j-carpentier/nova,alvarolopez/nova,Tehsmash/nova,eayunstack/nova,klmitch/nova,whitepages/nova,Juniper/nova,zhimin711/nova,ntt-sic/nova,hanlind/nova,NeCTAR-RC/nova,saleemjaveds/https-github.com-openstack-nova,gspilio/nova,ntt-sic/nova,jeffrey4l/nova,kimjaejoong/nova,OpenAcademy-OpenStack/nova-scheduler,vladikr/nova_drafts,plumgrid/plumgrid-nova,eonpatapon/nova,Yuriy-Leonov/nova,JianyuWang/nova,ted-gould/nova,Metaswitch/calico-nova,gooddata/openstack-nova,zzicewind/nova,orbitfp7/nova,joker946/nova,klmitch/nova,tangfeixiong/nova,vmturbo/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,dawnpower/nova,takeshineshiro/nova,TieWei/nova,mahak/nova,NeCTAR-RC/nova,JianyuWang/nova,alaski/nova,Yuriy-Leonov/nova,Francis-Liu/animated-broccoli,shootstar/novatest,adelina-t/nova,jianghuaw/nova,eayunstack/nova,virtualopensystems/nova,projectcalico/calico-nova,hanlind/nova,edulramirez/nova,gooddata/openstack-nova,dawnpower/nova,Triv90/Nova,yrobla/nova,iuliat/nova,petrutlucian94/nova_dev,zaina/nova,rahulunair/nova,rahulunair/nova,akash1808/nova_test_latest,sacharya/nova,leilihh/novaha,rrader/nova-docker-plugin,alexandrucoman/vbox-nova-driver,TwinkleChawla/nova,rahulunair/nova,leilihh/novaha,hanlind/nova,tudorvio/nova,viggates/nova,zzicewind/nova,CiscoSystems/nova,vmturbo/nova,Triv90/Nova,maelnor/nova,devoid/nova,leilihh/nova,alvarolopez/nova,DirectXMan12/nova-hacking,eharney/nova,TieWei/nova,tianweizhang/nova,ted-gould/nova,rajalokan/nova,projectcalico/calico-nova,silenceli/nova,cloudbase/nova,mikalstill/nova,tudorvio/nova,maheshp/novatest,ewindisch/nova,cloudbau/nova,berrange/nova,sridevikoushik31/nova,blueboxgroup/nova,takeshineshiro/nova,isyippee/nova,mgagne/nova,Yusuke1987/openstack_template,dstroppa/openstack-smartos-nova-grizzly,fnordahl/nova,yrobla/nova,dstroppa/openstack-smartos-nova-grizzly,mandeepdhami/nova,qwefi/nova,mahak/nova,tealover/nova,Stavitsky/nova,bclau/nova,tianweizhang/nova,sebrandon1/nova,CiscoSystems/nova,raildo/nova,phenoxim/nova,CEG-FYP-OpenStack/scheduler,cloudbase/nova-virtualbox,devoid/nova,thomasem/nova,watonyweng/nova,yosshy/nova,bigswitch/nova,Metaswitch/calico-nova,citrix-openstack-build/nova,double12gzh/nova,sridevikoushik31/nova,petrutlucian94/nova,whitepages/nova,mmnelemane/nova,yosshy/nova,raildo/nova,scripnichenko/nova,sridevikoushik31/nova,rickerc/nova_audit,akash1808/nova,petrutlucian94/nova_dev,watonyweng/nova,bgxavier/nova,dims/nova,rajalokan/nova,cernops/nova,yrobla/nova,sridevikoushik31/openstack,zaina/nova,sridevikoushik31/openstack,vmturbo/nova,phenoxim/nova,JioCloud/nova,eharney/nova,cyx1231st/nova,devendermishrajio/nova,Juniper/nova,shail2810/nova,gooddata/openstack-nova,gspilio/nova,tangfeixiong/nova,CCI-MOC/nova,maelnor/nova,mgagne/nova,leilihh/nova,yatinkumbhare/openstack-nova,blueboxgroup/nova,openstack/nova,thomasem/nova,redhat-openstack/nova,qwefi/nova,zhimin711/nova,CloudServer/nova,openstack/nova,bclau/nova,shahar-stratoscale/nova,MountainWei/nova,barnsnake351/nova,LoHChina/nova,JioCloud/nova_test_latest,apporc/nova,Tehsmash/nova,jeffrey4l/nova,noironetworks/nova,akash1808/nova_test_latest,SUSE-Cloud/nova,vladikr/nova_drafts,maheshp/novatest,sridevikoushik31/openstack,citrix-openstack-build/nova,scripnichenko/nova,Juniper/nova,JioCloud/nova_test_latest,tealover/nova,cloudbase/nova-virtualbox,rickerc/nova_audit,gooddata/openstack-nova,sridevikoushik31/nova,dstroppa/openstack-smartos-nova-grizzly,LoHChina/nova,imsplitbit/nova,saleemjaveds/https-github.com-openstack-nova,imsplitbit/nova,Francis-Liu/animated-broccoli,cyx1231st/nova,luogangyi/bcec-nova,ruslanloman/nova,nikesh-mahalka/nova,DirectXMan12/nova-hacking,felixma/nova,devendermishrajio/nova_test_latest,spring-week-topos/nova-week,felixma/nova,rajalokan/nova,petrutlucian94/nova,Stavitsky/nova,sebrandon1/nova,cloudbase/nova,angdraug/nova,cernops/nova,bigswitch/nova,mikalstill/nova,virtualopensystems/nova,CloudServer/nova,affo/nova,mahak/nova,TwinkleChawla/nova,maheshp/novatest,isyippee/nova,bgxavier/nova,angdraug/nova,nikesh-mahalka/nova,plumgrid/plumgrid-nova,OpenAcademy-OpenStack/nova-scheduler,barnsnake351/nova,shail2810/nova,shootstar/novatest,DirectXMan12/nova-hacking,viggates/nova,shahar-stratoscale/nova,rrader/nova-docker-plugin,silenceli/nova,mmnelemane/nova,alexandrucoman/vbox-nova-driver,alaski/nova,jianghuaw/nova,iuliat/nova,jianghuaw/nova,dims/nova,apporc/nova,Juniper/nova,devendermishrajio/nova_test_latest,Yusuke1987/openstack_template,jianghuaw/nova,JioCloud/nova,BeyondTheClouds/nova,BeyondTheClouds/nova,adelina-t/nova,CEG-FYP-OpenStack/scheduler,edulramirez/nova,joker946/nova,varunarya10/nova_test_latest,kimjaejoong/nova