enable setting the postgres user's password without resetting the password on every puppet run

this update is based on @mattyindustries idea from issue #92 https://github.com/puppetlabs/puppetlabs-postgresql/issues/92)

the underlying issue is that the default pg_hba.conf configuration doesn't allow TCP-based connections from the postgres user; unf. these are exactly the connections used by the password-testing functionality in postgresql::server::passwd

This worked fine on Puppet 3.1.1 on Ubuntu 12.04 LTS