[nova] [neutron] Include configuration directive for VIF/VETH MTU on havana

[SIGUSR2]

The openvswitch agent can be configured with a different MTU for the VIFs on the compute nodes. Also, nova can be configured with a different MTU on the VIF:VETH pairs. The configuration directives are: ovs_neutron_plugin.ini: veth_mtu = [MTU] nova.conf: network_device_mtu = [MTU]

In order to support a usage case where increased/adapted MTU is needed for the instances, I would suggest including these 2 configuration directives in the overridable attributes of the corresponding recipes.