Un-hardcoding domain/user name, removing domain for heat stack

The patch makes two things:
 1. Make it possible to un-hardcode domain/user name for heat
    stack;
 2. Remove deprecated and unused heat:server:stack_domain_admin:domain
    option from service level.

Change-Id: Ic8da908d1f40a96995ff1f3cb0385d9a279e604c
Related-Prod: PROD-25486
6 files changed