Respect the full tools list on InstanceConfig when building userdata config script.

[Eric Snow]

Currently the code in cloudconfig/userdatacfg_unix.go (and _win.go) only respects the first item in the list of tools available to it. At the moment this is fine since the code in the relevant methods does the right thing for the current Juju behavior. However, there is no guarantee that the provided tools list will continue to match the assumptions in the code.

[Andrew Wilkins (axwalk)]

It currently only does the right thing on Linux, not on Windows. Windows will continue to work so long as the machines are brought up on the same private network. I'm working on some changes to Azure that break this assumption, so I'll see how hard it is to fix.

[Andrew Wilkins (axwalk)]

This is not as straight forward as I had hoped. The change to userdata_unix.go to honour the URLs is trivial, but doing this exposes a latent bug elsewhere.

The tools URLs provided to cloudconfig have the model UUID in the URL, which we were previously ignoring. The tools catalogues are per-model, but we don't populate the hosted model catalogues when uploading to the controller.

So, I think we should have the tools download endpoint look in the model catalogue first, and fall back to the controller's catalogue. I'm not sure how deep that goes.

[Eric Snow (ericsnowcurrently)]

See:

  https://github.com/juju/juju/blob/master/cloudconfig/userdatacfg_unix.go#L332
  https://github.com/juju/juju/blob/master/cloudconfig/userdatacfg_win.go#L96