Fix AttributeError in admin_cleanup
Fix AttributeError in admin_cleanup

There will be an AttributeError in admin_cleanup when the user has no
tenant attached.

Change-Id: I1e34378dea3fbdf51c4218134d986bb8abf29d02
Closes-bug: 1363888

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