Description

This PR enables JIMM to use a default cloud when a user doesn't specify a cloud during model creation. The reasoning for the change is:

When running the Juju Terraform Provider’s test suite against JIMM it was found that JIMM behaves slightly differently from Juju when adding a model.
In Juju, when adding a model, if only 1 cloud is available, the user is not required to specifying a cloud and the available cloud will be used for model creation.
In JIMM, a cloud must always be specified otherwise an error is returned.

This duplicates #1325

Fixes CSS-10366

I also took the chance to move model related methods into a separate interface in jujuapi.

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

Test instructions

Notes for code reviewers