engine: Consider custom properties in Setup Networks

Added code to validate network custom properties as part of the Setup
Networks command, to mark networks as modified if their custom
properties have changed, and to serialize them for VDSM.

Persistence of the custom properties of a network is performed right
before sending the VDSM command; the alternative is to persist the
data when collecting network data after the VDSM command has finished
running, but at the moment VDSM doesn't report custom properties.

Change-Id: Ic607fb6073f2c4c0a4790763d9a09dc7879f11ca
Signed-off-by: Lior Vernia <lvernia@redhat.com>