RestClient: Don't hard code volume service name.
RestClient: Don't hard code volume service name.

Updates the common/rest_client keystone_auth function so that
it doesn't hard code the 'service_name' for the volume service.

The motivation here is to allow Tempest to run in more environments
where the volume service name may not always be 'cinder'.

Change-Id: I3b892781929642fd42a3dd3b4ba8d44486bfe275

pandeyop/tempest,izadorozhna/tempest,Mirantis/tempest,zsoltdudas/lis-tempest,afaheem88/tempest,hayderimran7/tempest,dkalashnik/tempest,NexusIS/tempest,neerja28/Tempest,NexusIS/tempest,izadorozhna/tempest,citrix-openstack-build/tempest,jaspreetw/tempest,Lilywei123/tempest,ebagdasa/tempest,masayukig/tempest,varunarya10/tempest,cloudbase/lis-tempest,rzarzynski/tempest,akash1808/tempest,dkalashnik/tempest,manasi24/jiocloud-tempest-qatempest,flyingfish007/tempest,adkerr/tempest,tonyli71/tempest,hpcloud-mon/tempest,bigswitch/tempest,zsoltdudas/lis-tempest,citrix-openstack-build/tempest,vedujoshi/tempest,jamielennox/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,Juniper/tempest,afaheem88/tempest_neutron,rzarzynski/tempest,Vaidyanath/tempest,hpcloud-mon/tempest,citrix-openstack/build-tempest,pczerkas/tempest,adkerr/tempest,roopali8/tempest,masayukig/tempest,alinbalutoiu/tempest,LIS/lis-tempest,cisco-openstack/tempest,bigswitch/tempest,danielmellado/tempest,tudorvio/tempest,yamt/tempest,CiscoSystems/tempest,tonyli71/tempest,armando-migliaccio/tempest,flyingfish007/tempest,neerja28/Tempest,sebrandon1/tempest,Vaidyanath/tempest,openstack/tempest,redhat-cip/tempest,Juniper/tempest,xbezdick/tempest,akash1808/tempest,alinbalutoiu/tempest,jaspreetw/tempest,varunarya10/tempest,redhat-cip/tempest,rakeshmi/tempest,Juraci/tempest,xbezdick/tempest,Mirantis/tempest,ntymtsiv/tempest,vedujoshi/tempest,cloudbase/lis-tempest,afaheem88/tempest,nunogt/tempest,pczerkas/tempest,itskewpie/tempest,manasi24/tempest,yamt/tempest,ntymtsiv/tempest,CiscoSystems/tempest,jamielennox/tempest,BeenzSyed/tempest,roopali8/tempest,pandeyop/tempest,vmahuli/tempest,manasi24/tempest,vedujoshi/os_tempest,Tesora/tesora-tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,itskewpie/tempest,JioCloud/tempest,rakeshmi/tempest,openstack/tempest,armando-migliaccio/tempest,vmahuli/tempest,eggmaster/tempest,danielmellado/tempest,Tesora/tesora-tempest,LIS/lis-tempest,afaheem88/tempest_neutron,vedujoshi/os_tempest,cisco-openstack/tempest,nunogt/tempest,queria/my-tempest,Lilywei123/tempest,eggmaster/tempest,sebrandon1/tempest,JioCloud/tempest,manasi24/jiocloud-tempest-qatempest,ebagdasa/tempest,citrix-openstack/build-tempest,Juraci/tempest,queria/my-tempest,tudorvio/tempest,BeenzSyed/tempest,hayderimran7/tempest