engine: Block change to old cluster if network properties set

Since we currently don't update VDSM upon cluster change in a host,
there is no way to wipe existing network custom properties on a host's
interfaces when moving to a cluster that doesn't support network
custom properties, and the operation should be blocked.

Change-Id: I16622d0374bde7ab2e440d81d82e830ad8d81949
Signed-off-by: Lior Vernia <lvernia@redhat.com>