fixtures: Unpin puppetlabs-mysql module

Since commit dd25406 of puppet-openstacklib we don't need to pin a
specific release of puppetlabs-mysql.
Migrate postgresql backend to use openstacklib::db::postgresql

Let migrate to the new openstacklib::db::postgresql helper for
postgresql backend. This commit also unpin postgresql fixture
(openstacklib support now the latest version of postgre module).

Implements: blueprint commmon-openstack-database-resource