networkTests: Fix operator precedence so a parameter to dnsmasq is not empty

--dhcp-option=3 (with the missing ,<router> part) forces dnsmasq not to pass
any gateway to DHCP clients, in contrast to sending the gateway of the system
dnsmasq is running on, when this parameter is not used.

No tests employ this functionality nowadays (they did before source route
testing) but let's be correct.

Change-Id: I04e2e1910fc527652f7e1767d48318686fb844b3
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35861
Reviewed-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>