Introduced by change-id: I413f77ce1dca59ae2c48ba743513ddf5584713e8
Nodegroup 'cluster' field has been changed to 'cluster_id' in
nailgun. So we need to adjust fuelclient accordingly.
Also includes some changes in nodegroups unit tests.
Change-Id: Iba3df02142279c14910578770187ac93157190c4
Closes-bug: #1518341