improve the pg_hba.conf whitelist hosts documentation

[paolantoni]

pg_hba.conf documentation may cause the failure of playbook RUNNING HANDLER [azavea.postgresql : Restart PostgreSQL] because the production.example file shows:

postgresql_hba_mapping:

- { type: "host", database: "all", user: "all", address: "{{ app_server_ip }}", method: "md5" }

- { type: "host", database: "all", user: "all", address: "{{ celery_server_ip }}", method: "md5" }

the subnet mask (mandatory) is not documented. Please update considering the subnet mask like here:

• { type: "host", database: "all", user: "all", address: "#.#.#.#/#", method: "md5" }