Juju register should offer a name for the controller

[Mark Shuttleworth]

I just tried to share a controller with another user. The registration process asked me to provide a name. I would expect it to offer up by default the same name that the sharing user had for that controller.

My expectation is that the controller would have a sense of its "preferred name" internally, and this would be sent to the registering client, to use as the prompted ("just hit enter") name for the controller there.

The client should note if there is a conflict with an existing, known controller name and warn appropriately.

For example:

  $ maas register JHGJH...JH

  Please set a name for this controller: gmaas

In other words, the name was offered up for me, I just have to hit ENTER.

If there is a conflict with a known controller name:

  $ maas register JHGJH...JH

  Warning, the controller proposed "gmaas" which clashes with an
  existing controller. The two controllers are entirely different.

  Please set a name for this controller:

In this second case I have entirely removed the suggested name, so hitting enter will fail. I chose this rather than trying to disambiguate the names ("gmaas-2").

[Horacio Durán (hduran-8)]

This change was implemented as suggested in the report, the controller name is now encoded in the registration string. The string becomes therefore larger, proportional to the model name lenght.
The PR for this fix is: https://github.com/juju/juju/pull/5745