custom_net_props: blacklist 'custom' for ifcfg configurator

The ifcfg configurator has a pass network options through that makes
those options that are not blacklisted reach the ifcfg files.
However these options must be textual and valid to initscripts to
be of any use. This patch blacklists custom as it is neither textual
nor valid initscripts.

Change-Id: Ifb4eba375ed64333b960c1cdb5e77bbd1c998c2d
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26655
Reviewed-by: Dan Kenigsberg <danken@redhat.com>