Implemented first draft for upgrade API.
Added cmd to set profile version.
Make sure to set the proper Plone site.
Implemented upgrade cockpit (WIP).
Added a basic test for ZooKeeper.
Declare set_profile_version method in IUpgradablePloneSite.
Refactored ZooKeeper for easier testing.
Added more ZooKeeper tests.
Cockpit: Handle KeyboardInterrupt more gracefully.
Renamed cluster.py to zookeeper.py
Cockpit: Moved upgrade runners to their own module.
More tests for ZooKeeper.