Add db::postgresql to openstacklib

The openstacklib::db::postgresql resource is a library resource that can be
used by nova, cinder, ceilometer, etc., rather than replicating equivalent
functionality across all of these modules. This resource is very simple, but
its addition will make maintenance and adding features much more
straightforward and consistent than implementing individually across modules.

openstacklib::db::postgresql uses the puppetlabs postgresql::server::db
resource to configure the database and user. openstacklib::db::postgresql
accepts a password_hash as a parameter and passes it to the
postgresql::server::db resource as the password parameter. While this seems to
conflict, the postgresql::server::db resource is actually using the password
parameter as a password hash.

Change-Id: I1446f37e7fba3305cff3eb3dd7ea4e7d5577eb4e
Implements: blueprint commmon-openstack-database-resource