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
diff --git a/metadata/service/server/single.yml b/metadata/service/server/single.yml
index 732a975..3cc782e 100644
--- a/metadata/service/server/single.yml
+++ b/metadata/service/server/single.yml
@@ -14,7 +14,7 @@
       stack_domain_admin:
         name: heat_domain_admin
         password: ${_param:heat_domain_admin_password}
-        domain: heat
+        stack_user_domain_name: heat_user_domain
       enabled: true
       region: RegionOne
       version: ${_param:heat_version}