Separate network node groups from clusters

Network node groups should be separated from clusters in Fuel.

Currently network node groups (which represent network isolated node groups, like racks) are attached to clusters and can't exist separately. This creates a bunch of problems, like 'default' node group, inability to bootstrap nodes from non-default node group without creating a cluster first, problems with discovered nodes from non-default node group when cluster is deleted, etc.

We can solve those problems by introducing new configuration unit in Fuel - node group. So user should be able to configure network node groups, bootstrap nodes and only then create clusters and assign nodes.