Ensure PostgreSQL is running before creating users

If PostgreSQL were manually stopped at any time post-install, a re-run
of the playbook would have failed because the postgresql_user module
would have had no database to connect to. Thus, make sure that the
service runs before we try to interact with the database.

roles/taiga-back/tasks/postgresql.yml

@@ -14,6 +14,17 @@
     - install
     - back-install
 
+- name: ensure that the database is running
+  become: true
+  become_user: root
+  service:
+    name: postgresql
+    state: started
+  tags:
+    - install
+    - back-install
+    - offline
+
 - name: create PostgreSQL user
   become: true
   become_user: postgres