Handle proxy environment variables for epel-release install
Handle proxy environment variables for epel-release install

Use yum_install() to support proxy environment variables when
epel-release package is installed from epel-bootstrap repo.

Change-Id: I45f7a38c6c71bbce07bd0d21c1ac8a75c04113cb
Closes-Bug: 1426221

openstack-dev/devstack,pczerkas/devstack,eharney/devstack,nawawi/openstack,noironetworks/devstack,kplus/devstack-moonshot,vsham20/devstack,dirkmueller/devstack,prithivm/branch01,Mahito/devstack,group-policy/devstack,HPCHub/devstack,varunarya10/devstack,eharney/devstack,sequenceiq/devstack,flavio-fernandes/devstack,pacinete/devstack,richliu/devstack,group-policy/devstack,mc2014/devstack,mmasaki/devstack,pombredanne/devstack,sc68cal/devstack,liuquansheng47/devstack,bq-xiao/devstack,bljgaurav/openstack-test,j4/devstack,bigswitch/devstack,vsham20/devstack,thomasem/devstack,kplus/devstack-moonshot,bigswitch/devstack,bljgaurav/openstack-test,parul24/devstack,costingalan/devstack,wenhuizhang/devstack,Mahito/devstack,openstack-dev/devstack,TwinkleChawla/devstack,herrsechs/devstack,HPCHub/devstack,mssumanth/devstack,wenhuizhang/devstack,samgoon/devstack,richliu/devstack,samgoon/devstack,BSJAIN92/OpenStack,upadhyay-prashant/devstack,atulpatil301/devstack,BSJAIN92/OpenStack,ozamiatin/devstack,nuxeh/devstack,rushiagr/devstack,LoHChina/devstack,rushiagr/devstack,LoHChina/devstack,pczerkas/devstack,olivierlemasle/devstack,flavio-fernandes/devstack,avvocatodemarchis/devstack,NaokiMiyata/devstack,bswartz/devstack,noironetworks/devstack,j4/devstack,nawawi/openstack,thomasem/devstack,dtroyer/devstack,TwinkleChawla/devstack,dirkmueller/devstack,sc68cal/devstack,CloudServer/devstack,atulpatil301/devstack,andrewrothstein/devstack,vishnugonela/devstack,liuquansheng47/devstack,varunarya10/devstack,costingalan/devstack,mmasaki/devstack,ozamiatin/devstack,avvocatodemarchis/devstack,CloudServer/devstack,NaokiMiyata/devstack,parul24/devstack,pacinete/devstack,neerja28/Devstack_GlusterFS,pombredanne/devstack,olivierlemasle/devstack,bq-xiao/devstack,vishnugonela/devstack,bswartz/devstack,mssumanth/devstack,upadhyay-prashant/devstack,andrewrothstein/devstack,neerja28/Devstack_GlusterFS,herrsechs/devstack,prithivm/branch01,dtroyer/devstack,sequenceiq/devstack,nuxeh/devstack,mc2014/devstack