Check for network verification status before deployment

We need to warn the user if network verification hasn't been perfromed or ended with error when he attempts to start deployment.

A new field "network_check_status" should be added to Cluster API. It should have 3 possible values: passed, failed and not_performed (default).

This field should be set by nailgun internally when some actions are performed:
1) The value should become "passed" when network check successfully passes
2) The value should become "failed" when network check fails
3) The value should become "not_performed" when current network verification status becomes invalid: in case of adding a new node, changing network settings, chaning interface configuration on one or more nodes, etc.
4) This field can also be set to "not_performed" directly by client (UI) when the user starts to edit configuration (discussable)

This field should NOT change its value when unsaved network settings are checked (clicking Verify Networks button without clicking Save Changes first)