User creation with cloud-init

[Martin Packman]

I proposed a branch this week that alters how juju uses cloud-init to
create the ubuntu user when provisioning a machine. I've had code
review, but would appreciate further feedback:

<http://reviews.vapour.ws/r/2860/>

The main aim here is to avoid depending on the /etc/cloud/cloud.cfg in
the image, but keep the logic unified across *nix systems. This
includes a CentOS behaviour change that I hope is okay, removing the
shell script use setup and just using cloud-config.

There are still a couple of things to be nervous about. The exisitng
container creation config is very much tied to ubuntu still, and I'm
not completely confident of stability across series for cloud-init
versions or default groups.

My intention is to get a reasonable amount of testing in advance of
landing the branch, but comments welcome as well.

Martin