Omit openstack/common/ in test coverage reports
Omit openstack/common/ in test coverage reports

These methods are tested in oslo-incubator not in rally,
so we should just ignore test covarage of them.

Change-Id: I8fb6a4be1968653ef982f60c896dcc6a2bf65625

ytsarev/rally,shdowofdeath/rally,paboldin/rally,eonpatapon/rally,aplanas/rally,go-bears/rally,afaheem88/rally,yeming233/rally,redhat-openstack/rally,vefimova/rally,ytsarev/rally,vganapath/rally,vefimova/rally,eayunstack/rally,vganapath/rally,varunarya10/rally,cernops/rally,eayunstack/rally,group-policy/rally,paboldin/rally,amit0701/rally,pandeyop/rally,varunarya10/rally,yeming233/rally,vponomaryov/rally,shdowofdeath/rally,afaheem88/rally,openstack/rally,aplanas/rally,gluke77/rally,gluke77/rally,group-policy/rally,pyKun/rally,vganapath/rally,aforalee/RRally,go-bears/rally,openstack/rally,vponomaryov/rally,amit0701/rally,redhat-openstack/rally,aforalee/RRally,vganapath/rally,eonpatapon/rally,pandeyop/rally,paboldin/rally,pyKun/rally,gluke77/rally,group-policy/rally,amit0701/rally,eayunstack/rally,openstack/rally,openstack/rally,gluke77/rally,cernops/rally