Add tempest users to heat_stack_owner role

Any user creating heat stacks needs to be added to this role.  Doing
this resolves some of the failing orchestration tempest tests.

(cherry picked from commit 13dfb3834188e9ed6017e89c6be1c59f1487d493)
Merge pull request #515 from mattt416/proposed/juno_346

Add tempest users to heat_stack_owner role