Make Admin and Private non-dedicated

Admin and Private can be paired with other networks now.
Private acts as untagged network.
Network groups' "meta" is exposed to UI.
"allowed_networks" model is removed.
Tests are added.

Implements: blueprint get-rid-of-networking-limitations
Implements: blueprint expose-networks-meta-from-nailgun-to-ui

Change-Id: Ic3383a2da87a34259463c2a2c97d05b20a7fc998