Set neutron MTU according to actual value

When deployed on OCP with default MTU settings actual MTU is limited to 1400 while neutron default is 1500.
Wrong MTU setting causes failures during some tests execution.

examples/dt/uni06zeta/control-plane/service-values.yaml

@@ -116,6 +116,7 @@ data:
       type_drivers = geneve,vlan
       tenant_network_types = geneve,vlan
       extension_drivers = qos,port_security,dns_domain_ports
+      path_mtu = 1400
       [ml2_type_geneve]
       vni_ranges = 1:65536
       max_header_size = 38