Allow multiple networks to be created per cluster

Added a NodeGroup model. NodeGroups can be created and modified
via the Fuel API. Clusters are created with a default node group
and the cluster networks are tied to that default node group.

Added API handler for creating new NetworkGroups.

Implements: blueprint multiple-cluster-networks
Closes-bug: #1272142
Change-Id: Ic30a1b46112599022645b06f7bef2223ff4c6475