Call _post_downgrade_### after downgrade migration is run
Call _post_downgrade_### after downgrade migration is run

Fixes bug 1195470

Tests are currently only run after a upgrade during the database migration
unit tests and never during a downgrade. This change calls an optional
_post_downgrade_### method after running the downgrade migration to allow
it to be tested as well.

Change-Id: I592424b464b91c2be9e253402986e673ce33c8d0

citrix-openstack-build/glance,jumpstarter-io/glance,klmitch/glance,cloudbau/glance,saeki-masaki/glance,rickerc/glance_audit,takeshineshiro/glance,tanglei528/glance,darren-wang/gl,kfwang/Glance-OVA-OVF,redhat-openstack/glance,wkoathp/glance,paramite/glance,rickerc/glance_audit,klmitch/glance,paramite/glance,openstack/glance,stevelle/glance,akash1808/glance,akash1808/glance,dims/glance,wkoathp/glance,scripnichenko/glance,citrix-openstack-build/glance,cloudbau/glance,dims/glance,openstack/glance,darren-wang/gl,saeki-masaki/glance,ozamiatin/glance,vuntz/glance,stevelle/glance,kfwang/Glance-OVA-OVF,takeshineshiro/glance,JioCloud/glance,JioCloud/glance,rajalokan/glance,sigmavirus24/glance,ntt-sic/glance,ozamiatin/glance,rajalokan/glance,redhat-openstack/glance,SUSE-Cloud/glance,scripnichenko/glance,ntt-sic/glance,sigmavirus24/glance,vuntz/glance,jumpstarter-io/glance,tanglei528/glance,openstack/glance,SUSE-Cloud/glance