Configures Heat to use Trusts by default

- This option has existed since Havana release and the Heat team is moving
  towards making this the default.
- Sets deferred_auth_method=trusts and trusts_delegated_roles=heat_stack_owner
  in heat.conf
- Creates heat_stack_owner role.
- Assigns heat_stack_owner role to admin and demo users.

Change-Id: I2e42d82f1bde019a3c2f0e52ab677fdf457d29a4
Fixes: rhbz#1108155
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
Merge "Configures Heat to use Trusts by default" into icehouse