Add support for custom PostgreSQL host-based authentication

This changeset adds support for custom PostgreSQL host-based authentication via Ansible variables. `postgresql_hba_mapping` is a list of maps that contain the following attributes:

- type
- database
- user
- address
- method

Inside a playbook, PostgreSQL HBA could be customized using:

```
vars:
  postgresql_listen_addresses: "*"
  postgresql_hba_mapping:
    - { type: "host", database: "all", user: "all", address: "0.0.0.0/0", method: "md5" }
```