Validate created/updated stacks in StackResource

StackResource.create_with_template and update_with_template now validate
the resulting stacks.

Change-Id: I52a81edb2af6bee70146f6dbd2179900e580d69b
Use correct actions for StackResource

To perform actions on a stack, we should use the action constants defined
in the Stack, not in another object (and especially not in a Resource).

Change-Id: I3044f6286fc2690c121a3446f42a6896b3ed3271
Merge "Validate created/updated stacks in StackResource"
Merge "Use correct actions for StackResource"