breaking API change: controller vs model API connections

[roger peppe]

There is a PR that's soon to land that will change the Juju
API so that controller-level RPC calls can no longer
be executed on a model-specific connection.

https://github.com/juju/juju/pull/5986

This is a breaking API change.

The affected facades are the following:

    AllModelWatcher
    Cloud
    Controller
    MigrationTarget
    ModelManager
    UserManager

Note that this is made possible by Andrew Wilkins'
recent change to ModelManager.DestroyModel, which
means that it's possible to specify the destroyed model in the
API call rather than implicitly in the API connection.

Note: to make a controller-only connection, connect
to /api endpoint rather than the /model/:uuid/api endpoint.
Note that this is another change: the preferred endpoint
has changed from / to /api for consistency with the model
endpoint (and to give freedom to redirect to an HTML index page
or similar in the future). Although / is still supported for
the time being, it should be considered deprecated once
https://github.com/juju/juju/pull/5988 lands.

  cheers,
    rog.