Use the Ansible module postgresql_privs_module to manage user privileges

http://docs.ansible.com/ansible/postgresql_privs_module.html

It'd be more expressive/convenient to use this rather than the `postgresql_user` module for managing user privileges: https://github.com/ANXS/postgresql/blob/master/tasks/users_privileges.yml#L4

It's an Ansible core module, so should be supported by them.
