schema: publish custom network properties

This patch makes part of the formal api a network attribute named
'custom' that allows the user to pass a free-form dictionary of
attributes that may be used by vdsm or its hooks to perform actions.

Change-Id: Ia8461cf9daea7c4b76cbfde05b8e85aeea4195b2
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26671
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>