Support JAAS as default controller if only credentials are provided

[johnsca]

With the public beta of JAAS available, we could skip the register-controller requirement now and just use JAAS (for the three main clouds, at least).

Things to consider:

• Deciding when to use JAAS as one of the default "all controllers" option, especially if an explicit controller for that cloud is registered
• When adding a model using JAAS, we have to be explicit about the cloud & region
• We'd need a charm store login to use JAAS, but we probably need that anyway
• LXD and any other cloud would still need to be explicitly registered