Add cover to gitignore
Add cover to gitignore

Test coverage reports are stored in cover directory. It should be
ignored by git.

Change-Id: I35fb4721512859be3b98d70bbab1a8988d4f64e8

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