Allow admin role name to be changed

The os_keystone role has a variable 'keystone_role_name' which
allows the name of the 'admin' role to be changed to something
else.

This patch implements the ability to use the same variable name
when adding the heat admin role.

Change-Id: I8873986086438434b409fb3129f919aa1233663e

tasks/heat_domain_setup.yml

@@ -69,7 +69,7 @@
     login_password: "{{ keystone_auth_admin_password }}"
     login_project_name: "{{ keystone_admin_tenant_name }}"
     user_name: "{{ heat_stack_domain_admin }}"
-    role_name: "admin"
+    role_name: "{{ keystone_role_name | default('admin') }}"
     domain_name: "{{ heat_stack_user_domain_name }}"
     insecure: "{{ keystone_service_adminuri_insecure }}"
   register: add_service