net tests: allowDhcp is no-op if there is no firewall

firewall.allowDhcp is intended to allow DHCP traffic to the tested
interface. With this patch, we assume that if we find no firewall
service running, DHCP is already allowed.

This change allows DHCP-related tests to run when no firewall is
running on host.

Change-Id: I7de84c081ab49cca3a7f358cdffd25a21c08ea7b
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35911
Tested-by: Ido Barkan <ibarkan@redhat.com>