Add puppet_openstack_builder to gerritbot config

The #puppet-openstack IRC channel currently isn't being
notified about new patches to the stackforge/puppet_openstack_builder
project because gerritbot hasn't been told to notify it.  This
change adds a notification to gerritbot's config for the project.

Change-Id: I68fdace53791244102c7ebbda0dd3690082b6843
Merge "Add puppet_openstack_builder to gerritbot config"