clean dhclient processes that are left after functional tests

using a context manager to kill dhclient process that
outlives the functional test

Change-Id: I458aa38415c697d3863e173444ff921d759166a2
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34366
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>