Add Glance Cache Pruner to Control Nodes

We currently don't set up a glance cache pruner cronjob on nodes on which we run glance, which can lead to disk space being used up. Running glance cache pruner is recommended practice, so we should set up a cronjob for this.

For more information on glance cache pruner, refer to: http://docs.openstack.org/developer/glance/cache.html