postgresql_user: Add support for PUBLIC

This change enables users to add and remove permissions to all roles,
using the PUBLIC user. This is equivalent to using the PUBLIC keyword
in GRANT and DENY postgres SQL commands.

For example, see: <http://www.postgresql.org/docs/current/interactive/sql-grant.html>

Fixes #1833
Add documentation about PUBLIC user
Merge pull request #2128 from lorin/postgres-public

postgresql_user: Add support for PUBLIC