Some network-related code now uses flags from meta

1. Network metadata is now accessible from 'meta' field of NetworkGroup model.
2. Three network metadata flags added.

Flags description: https://etherpad.openstack.org/p/nailgun-network-group-flags

Change-Id: Ia8494c98086b29692b70b0f7a939db2ab91b2dea
Merge "Some network-related code now uses flags from meta"