Simplify add_user_to_group function
Simplify add_user_to_group function

Current SLE12 and openSUSE13.X versions can handle usermod's '-a' and '-G'
switches so remove the special case.

Change-Id: If0f1390a0eb8f41ffffca74525a4648cfe8ea61d

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