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
Merge "Clean up VPC unit tests"