Juju instance names could be more compact

In doing a Juju bootstrap on LXD I saw this:

juju-e8bb7715-e092-4003-88d3-99ae78e17d2c-machine-0

In truth, it's unlikely to be a risk to use even a tiny portion of that random namespace. For example, if we just used the last 12 random characters in that name we'd have:

   juju-99ae78e17d2c-machine-0

That's still several hundred thousand billion Juju models we could have without a clash :)

If we have the ability to detect collisions, we could reduce it even to the last 5 characters in the model UUID. In other words, take the last 5 characters of the model and use that, unless it collides, in which case add a character (so use the last 6).

We'll need to store the full model somewhere in the metadata for the machine, I guess, rather than in the name, if we do this.

Finally, I think the "-machine" is redundant too, given the juju- prefix. Perhaps just this would be sufficient:

 juju-43d21-0
 juju-43d21-1
 juju-43d21-2
 juju-43d21-3
 ...