Increase flexibility of stackrc repo config.
Increase flexibility of stackrc repo config.

 * Offline use of devstack previously required defining individual
   repo overrides.  This change maintains support for individual
   overrides while adding the ability to override GIT_BASE.

Change-Id: I4ca8b8e69c0fb05a7c9cf2fd4643eac2f7643aa9

pacinete/devstack,liuquansheng47/devstack,olivierlemasle/devstack,affo/devstack,group-policy/devstack,neerja28/Devstack_GlusterFS,nawawi/openstack,vishnugonela/devstack,dtroyer/devstack,eshnil2000/devstack,thomasem/devstack,bljgaurav/openstack-test,innodee/openstackk,affo/devstack,pacinete/devstack,HPCHub/devstack,BSJAIN92/OpenStack,avvocatodemarchis/devstack,LoHChina/devstack,openstack-dev/devstack,yamt/devstack,noironetworks/devstack,wenhuizhang/devstack,sc68cal/devstack,nuxeh/devstack,prithivm/branch01,pczerkas/devstack,thomasem/devstack,NaokiMiyata/devstack,flavio-fernandes/devstack,Mahito/devstack,nuxeh/devstack,rushiagr/devstack,liuquansheng47/devstack,mssumanth/devstack,bswartz/devstack,CiscoSystems/devstack,pombredanne/devstack,CloudServer/devstack,j4/devstack,TwinkleChawla/devstack,armando-migliaccio/devstack,wenhuizhang/devstack,richliu/devstack,mc2014/devstack,tianweizhang/devstack,promptworks/devstack,sequenceiq/devstack,pczerkas/devstack,nawawi/openstack,bq-xiao/devstack,BSJAIN92/OpenStack,dtroyer/devstack,parul24/devstack,noironetworks/devstack,atulpatil301/devstack,atulpatil301/devstack,HPCHub/devstack,dirkmueller/devstack,rushiagr/devstack,group-policy/devstack,ozamiatin/devstack,prithivm/branch01,upadhyay-prashant/devstack,virtualopensystems/devstack,mmasaki/devstack,CloudServer/devstack,bigswitch/devstack,tianweizhang/devstack,eharney/devstack,deepakselvaraj/devstack,vsham20/devstack,openstack-dev/devstack,kplus/devstack-moonshot,pombredanne/devstack,varunarya10/devstack,avvocatodemarchis/devstack,herrsechs/devstack,mc2014/devstack,richliu/devstack,srics/devstack,sc68cal/devstack,Mahito/devstack,ozamiatin/devstack,LoHChina/devstack,Juniper/devstack,samgoon/devstack,eharney/devstack,TwinkleChawla/devstack,sequenceiq/devstack,jamielennox/devstack,flavio-fernandes/devstack,parul24/devstack,varunarya10/devstack,olivierlemasle/devstack,andrewrothstein/devstack,vsham20/devstack,upadhyay-prashant/devstack,samgoon/devstack,mmasaki/devstack,vishnugonela/devstack,dirkmueller/devstack,williamthegrey/devstack,costingalan/devstack,Yuriy-Leonov/devstack,j4/devstack,bigswitch/devstack,costingalan/devstack,mssumanth/devstack,promptworks/devstack,herrsechs/devstack,Millnert/contrail-devstack,kplus/devstack-moonshot,neerja28/Devstack_GlusterFS,inkerra/devstack,NaokiMiyata/devstack,JioCloud/devstack,andrewrothstein/devstack,sstrato/devstack,shootstar/devstacktest,bljgaurav/openstack-test,bswartz/devstack,bq-xiao/devstack