Add glance cache pruner job

On nodes where we set up glance, we currently don't set up a
glance cache pruning job.  This is a recommended practice as
it can allow glance images to take up more disk space than
they should be allowed to by the image_cache_max_size setting
(refer to http://docs.openstack.org/developer/glance/cache.html).
This patch adds glance::cache::pruner to the glance_all
class group so that a pruner job gets added to nodes running
glance.

Change-Id: I9cf46057f7cf436a54c121b5364a2d17ebc9fbb8
Implements: blueprint add-glance-cache-pruner
Merge "Add glance cache pruner job"