update_cluster fixes

Correct code that identifies proxy cartridges

Add support for rolling back update_cluster

Move gear_registry.* files underneath a new gear_registry directory

Change gear_registry.* files to be owned by root

Add backup/restore_from_backup methods to GearRegistry

Call update_cluster in Application#calculate_gear_remove_ops so when
deleting a gear via oo-admin-ctl-app it will update the gear registry
and proxy info
Merge pull request #33 from ncdc/fix-update-cluster

update_cluster fixes