[neutron] Allow Neutron dhcp agent to use dnsmasq_config_file

[odyssey4me]

When using gre tunnelling with Neutron you may find errors like this on the L3 Gateway in syslog:

Dec 10 12:53:50 l3gateway.lab kernel: [3996133.097828] openvswitch: br-trunk0: dropped over-mtu packet: 1502 > 1500

You may also notice some packet loss between instances. On busy hosts we've seen 10-15% dropped udp packets!

The root cause is that ovs is adding tagging information to the packets, so the resulting packet that needs to be tunnelled ends up larger than the MTU and is dropped. More information is here: http://openstack.redhat.com/forum/discussion/comment/1565

The workaround is to ensure that Neutron delivers a smaller than standard MTU size via DHCP or to configure all instance images to have a smaller MTU size preconfigured. There are more details here: http://techbackground.blogspot.com/2013/06/dnsmasq-logging-and-options-for-quantum.html

In order to facilitate this process it would be prudent to configure the nova-network cookbook to allow the Neutron DHCP Agent to make use of the dnsmasq_config_file option and to pre-populate the MTU option into that config file.

[JCallicoat]

This will also allow for hooking into IP allocate and release events via the dnsmasq dhcp-script option. We already have the attributes ( rcbops-cookbooks/nova@b5aca4f96a ) but they are not being used in the neutron configs.

[JCallicoat]

rcbops-cookbooks/nova-network/pull/108