Refactored restage test to be transactional

Signed-off-by: Chris Adams <cadams@pivotallabs.com>
Added ModelManager to handle cleaning up models

- For some reason, transactional mode wasn't correctly cleaning up the db

Signed-off-by: Chris Adams <cadams@pivotallabs.com>
Destroying Stacks instead of deleting them

- Destroy deletes associated dependencies too (to ensure constraint
  compliance)

Signed-off-by: Chris Adams <cadams@pivotallabs.com>