Make it possible to pxeboot ports.

Nova baremetal needs to send PXE boot instructions to machines. It currently runs its own dnsmasq process to accomplish this, but this is listening on the same broadcast domain as the quantum dhcp-agent's dnsmasq (or equally that of nova-network). This makes booting very unreliable.

I discussed this on the list a few times - http://lists.openstack.org/pipermail/openstack-dev/2013-January/004363.html is a decent summary / entry point.

What I propose to do is to add a pxeboot extension (extensions?) to quantum that allow nova to specify the pxe boot details when it creates or updates a port. Guidance on exactly what that looks like in code appreciated :)

Overall arc looks like:
 - teach quantum ports to store pxe boot details
 - teach the dhcp agent to configure that (see list reference for how that should look on disk)
 - teach quantumclient to drive this api
 - teach nova to ask the hypervisor for pxe details as part of network setup

We can assume that any pxe details for a port are reachable via the network the port is on (e.g. it is not quantums problem to ensure a sane setup).