Cluster object

Related to blueprint nailgun-objects-flow

Change-Id: I3dd4c13c8f269cc81a97aa8a3803cd036f56e9bf
Cluster helpers now in object & tests

Related to blueprint nailgun-objects-flow

Moved all helpers from Cluster model to object

Removed unnecessary "with" wrappers for transactions

Change-Id: Id4a15c59eb6b10b7573ce0c559c59f6d99f54002
Merge "Cluster object"
Merge "Cluster helpers now in object & tests"