Stop making the database migration backend lazy pluggable
Stop making the database migration backend lazy pluggable

We only have one database migration backend, and this should
not be an extension point that we make public.

LazyPluggable migration backend was added early on but never
used in tree: 4f5c0c64ec9d397048dfd7b8d5c007ec0fa39ec5

Change-Id: I37a5aa58d81b575bf3e4dfb9b6f8dcca931a889e

blueboxgroup/nova,JioCloud/nova,ruslanloman/nova,rahulunair/nova,zaina/nova,NeCTAR-RC/nova,dawnpower/nova,CEG-FYP-OpenStack/scheduler,rahulunair/nova,nikesh-mahalka/nova,Francis-Liu/animated-broccoli,varunarya10/nova_test_latest,gooddata/openstack-nova,raildo/nova,tangfeixiong/nova,rajalokan/nova,BeyondTheClouds/nova,hanlind/nova,devendermishrajio/nova,gooddata/openstack-nova,edulramirez/nova,devendermishrajio/nova,joker946/nova,barnsnake351/nova,scripnichenko/nova,yosshy/nova,yosshy/nova,Yusuke1987/openstack_template,mmnelemane/nova,Stavitsky/nova,raildo/nova,akash1808/nova,TwinkleChawla/nova,jianghuaw/nova,alaski/nova,jianghuaw/nova,hanlind/nova,felixma/nova,mandeepdhami/nova,joker946/nova,jeffrey4l/nova,jianghuaw/nova,sebrandon1/nova,bgxavier/nova,mandeepdhami/nova,mahak/nova,openstack/nova,gooddata/openstack-nova,JioCloud/nova_test_latest,dims/nova,isyippee/nova,klmitch/nova,alexandrucoman/vbox-nova-driver,ted-gould/nova,rahulunair/nova,dawnpower/nova,Francis-Liu/animated-broccoli,cyx1231st/nova,alvarolopez/nova,Juniper/nova,apporc/nova,mmnelemane/nova,JianyuWang/nova,devendermishrajio/nova_test_latest,jeffrey4l/nova,cernops/nova,mikalstill/nova,JioCloud/nova_test_latest,vmturbo/nova,akash1808/nova_test_latest,cyx1231st/nova,cernops/nova,noironetworks/nova,akash1808/nova,mikalstill/nova,shail2810/nova,bigswitch/nova,ruslanloman/nova,tudorvio/nova,shail2810/nova,watonyweng/nova,takeshineshiro/nova,vmturbo/nova,orbitfp7/nova,rajalokan/nova,mikalstill/nova,Juniper/nova,belmiromoreira/nova,zzicewind/nova,zzicewind/nova,BeyondTheClouds/nova,thomasem/nova,belmiromoreira/nova,eonpatapon/nova,orbitfp7/nova,sebrandon1/nova,phenoxim/nova,whitepages/nova,tealover/nova,cernops/nova,iuliat/nova,kimjaejoong/nova,LoHChina/nova,ted-gould/nova,isyippee/nova,tangfeixiong/nova,watonyweng/nova,Juniper/nova,kimjaejoong/nova,yatinkumbhare/openstack-nova,eonpatapon/nova,fnordahl/nova,phenoxim/nova,CloudServer/nova,cloudbase/nova,iuliat/nova,klmitch/nova,NeCTAR-RC/nova,klmitch/nova,alexandrucoman/vbox-nova-driver,hanlind/nova,double12gzh/nova,nikesh-mahalka/nova,tealover/nova,LoHChina/nova,Tehsmash/nova,sebrandon1/nova,thomasem/nova,Juniper/nova,CCI-MOC/nova,zhimin711/nova,tudorvio/nova,affo/nova,affo/nova,vmturbo/nova,akash1808/nova_test_latest,JioCloud/nova,cloudbase/nova-virtualbox,rajalokan/nova,petrutlucian94/nova,vmturbo/nova,jianghuaw/nova,edulramirez/nova,petrutlucian94/nova,noironetworks/nova,TwinkleChawla/nova,Stavitsky/nova,BeyondTheClouds/nova,CCI-MOC/nova,dims/nova,blueboxgroup/nova,MountainWei/nova,varunarya10/nova_test_latest,Tehsmash/nova,alvarolopez/nova,MountainWei/nova,bgxavier/nova,bigswitch/nova,yatinkumbhare/openstack-nova,mahak/nova,cloudbase/nova,zaina/nova,devendermishrajio/nova_test_latest,j-carpentier/nova,JianyuWang/nova,CEG-FYP-OpenStack/scheduler,zhimin711/nova,cloudbase/nova,rajalokan/nova,mahak/nova,takeshineshiro/nova,Yusuke1987/openstack_template,barnsnake351/nova,gooddata/openstack-nova,CloudServer/nova,adelina-t/nova,klmitch/nova,alaski/nova,cloudbase/nova-virtualbox,whitepages/nova,double12gzh/nova,scripnichenko/nova,apporc/nova,openstack/nova,fnordahl/nova,felixma/nova,adelina-t/nova,openstack/nova,j-carpentier/nova