nettests: use a constant dict NOCHK

The options {'connectivityCheck': False} are very common, lets make it
easier to use them.

Change-Id: I06bed9ca60272d94d157f278fdf23e354dbf29dd
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24166
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>