Added postgresql::backup defined resource

Also added a new hash parameter, postgresql::backups, to the postgresql class,
so that postgresql::backup defined resources can realized by the main class.
This is useful for ENCs that don't support defined resources (e.g. Foreman) but
do support hash-type parameters. This change also adds explicit support for
Ubuntu 12.04 and cleans up postgresql::params a bit.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>