Add glance::db::postgresql class.

This allows Glance to be used with PostgreSQL.

Requires the puppetlabs/postgresql module. NOTE: I did add this
dependency to fixtures but did not add it to the Modulefile due
to the fact that the postgresql::python module hasn't been
released to puppetforge yet (although it is committed and tests
pass).

Change-Id: I3522d50935c81c8859c9f597df64aa0af6ca4e71