Clean up VPC unit tests
Clean up VPC unit tests

 - Ensure that stacks are cleaned up on failure.
 - Add a test for the case where an something other than a valid reference
   is passed as the GroupSet.
 - Add a test for the case where no GroupSet is passed.

Ref bug #1164913

Change-Id: I7637ed7969f23662643756da4a49627a2b40de20
pshchelo/heat,steveb/heat,cwolferh/heat-scratch,ntt-sic/heat,cryptickp/heat,rdo-management/heat,NeCTAR-RC/heat,jasondunsmore/heat,jasondunsmore/heat,JioCloud/heat,cwolferh/heat-scratch,dragorosson/heat,miguelgrinberg/heat,cryptickp/heat,steveb/heat,takeshineshiro/heat,citrix-openstack-build/heat,noironetworks/heat,rickerc/heat_audit,redhat-openstack/heat,rh-s/heat,pratikmallya/heat,dims/heat,rdo-management/heat,ntt-sic/heat,varunarya10/heat,dragorosson/heat,miguelgrinberg/heat,dims/heat,srznew/heat,NeCTAR-RC/heat,rickerc/heat_audit,pshchelo/heat,takeshineshiro/heat,gonzolino/heat,redhat-openstack/heat,JioCloud/heat,rh-s/heat,maestro-hybrid-cloud/heat,openstack/heat,srznew/heat,varunarya10/heat,noironetworks/heat,pratikmallya/heat,maestro-hybrid-cloud/heat,openstack/heat,gonzolino/heat,citrix-openstack-build/heat