Add .stackenv to gitignore.
Add .stackenv to gitignore.

Change-Id: I39e91aaf5e9ff29b025fd5a1aa74bad01c5e5bdd

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