Allow filtering of stacks by action

Currently users can already filter by status.  Since status and action
are so closely tied, it is valuable to be able to filter by action as
well.  As a result, users can filter stacks in a more meaningful way.
For example, /stacks?action=CREATE&status=IN_PROGRESS

Implements: blueprint filter-by-action
Change-Id: I58bf53373e0a9231d198d8fb62e76b1373587b87
Make template validation uniform

There were two different behaviours during template validation and creation
stack (with template validation). In first case error was raised if
template did not have any resources. However user could create stack
without error using this such kind of template.
This patch make behaviour equal for both variant and gives ability to
create and successfully validate stack with template which does not have
resources.

Change-Id: I4dc30ccdc45e0fa7ff678b434106e056dba5f3b4
Closes-Bug: #1278090
Merge "Allow filtering of stacks by action"
Merge "Make template validation uniform"