r/postgresql-server: Set become on postgres tasks

Tasks that must run as the _postgres_ user need to explicity enable
`become`, in case it is not already enabled at the playbook level.  This
can happen, for example, when the playbook is running directly as root.

roles/postgres-exporter/tasks/main.yml

@@ -27,6 +27,7 @@
   - user
 
 - name: ensure postgres-exporter postgresql role exists
+  become: true
   become_user: postgres
   postgresql_user:
     name: postgres-exporter
@@ -34,6 +35,7 @@
   tags:
   - pguser
 - name: ensure postgres-exporter postgresql is in pg_monitor group
+  become: true
   become_user: postgres
   postgresql_membership:
     groups: