Idempotent nova db privilege grants

The nova_api_galera_user is granted privileges for both the
nova_api_galera_database and nova_cell0_database. Use append_privs for
both of these tasks so that privileges aren't being overwritten and
re-added on each run of the playbook.

Change-Id: Ide9d44582bd1ecc126d198089ae9879163f61808

playbooks/os-nova-install.yml

@@ -63,6 +63,7 @@
         password: "{{ nova_api_container_mysql_password }}"
         login_host: "{{ nova_api_galera_address }}"
         db_name: "{{ nova_api_galera_database }}"
+        db_append_privs: "yes"
       run_once: yes
 
     - name: Configure MySQL user (nova-placement)