tests: Add and use assertRuleExists and assertRuleDoesntExist

DHCPv6-enabled testSetupNetworksAddDelDhcp needs a better diagnosis.

Change-Id: Icc292135482da28065d0d59fc3a67cddc3a2f2f9
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34991
Reviewed-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>