Allow db user name differing from db name

For some services, the name of the database user may differ from the
name of the database that user needs access to. Add a 'db_user' variable
for use in this case, defaulting to 'db_name' to maintain current
functionality.

Change-Id: Iff2d736415cfbe3e1ec60aa370cc30bd8610541d

create-grant-db.yml

@@ -26,7 +26,7 @@
     login_user: "root"
     login_password: "secrete"
     login_host: "127.0.0.1"
-    name: "{{ db_name }}"
+    name: "{{ db_user | default(db_name) }}"
     password: "{{ db_password }}"
     host: "{{ item }}"
     state: "present"