WIP: More resilient bridge creation script for MAAS

[dimitern]

Improved juju-br0 creation script for MAAS to work with bonds, aliases,
and IPv6 config. It became almost 200 lines of bash, which I'm not happy
about, that's why it's WIP (and there are no tests yet).

We might need to do the whole thing in python and properly test it in
isolation. So far I've manually tested this on MAAS 1.9alpha4 with many
different network setups for nodes: 1 NIC, 2 NICs, aliases, bonds, bonds
and aliases, IPv6 on one NIC, etc. As long as the node networking is
done properly (i.e. when deploying via MAAS without Juju the node is
accessible), the bridge is created fine and both KVM and LXC machines
are deployed fine with the expected addresses and are accessible.

(Review request: http://reviews.vapour.ws/r/2937/)

[dimitern]

Closing this, as @frobware took it over and will propose it when done.