allow setting fixed MAC addresses for juju created LXC containers

Our environment requires stable IP addresses for LXC containers deployed to MAAS managed physical machines via juju.

We are smooshing OpenStack infrastructure components (glance, cinder, rabbitmq-server, swift-proxy, ...) to 3 units each on 3 physical hosts for HA.

HA enabled services using the hacluster charm require that we specify a virtual IP via config, but the IP of an LXC container comes from a pool no matter if MAAS is used for DHCP/DNS management or not. MAC addresses are randomized by default as well, using /etc/lxc/default.conf on the host machine.

I suggest allowing the MAC address to be pre-allocated via config, such that IP and hostname can be pre-allocated via DHCP/DNS as well.

References:

http://manage.jujucharms.com/charms/trusty/hacluster
https://bugs.launchpad.net/ubuntu/+source/maas/+bug/1274947
http://manpages.ubuntu.com/manpages/trusty/man5/lxc.container.conf.5.html

$ lsb_release -rd
Description: Ubuntu 12.04.4 LTS
Release: 12.04

juju-core version: 1.18.4
Target distro: trusty