Add collection count to stack list

Add an optional parameter ``with_count`` to the stack_list API to
allow the inclusion of the collection count in the body of the response

When passing the parameter, the response looks like:
{
  'stacks': [...],
  'count': 42
}

blueprint: count-stacks
Change-Id: I560cfff9d327db9b325e7cf4786bd2fcd71d47b2