feature/rework model defaults cli

[reedobrien]

Update tests accordingly.

Refs: juju-model-defaults-collapse

QA:

1. All unit tests pass
2. juju bootstrap reed-aws-west-1 awstest # with cloud like http://paste.ubuntu.com/23080517/
3. juju model-defaults # verify that the output matches what you'd expect from the config above.
4. juju model-defaults image-stream=develop test-mode=true # ensure settings are updated with juju model-config
5. juju model-defaults --reset image-stream test-mode # ensure they are unset
6. juju model-defaults image-stream
   ATTRIBUTE DEFAULT CONTROLLER
   image-stream released -
7. juju model-defaults --format=json image-stream
   {"image-stream":{"default":"released"}}
8. Maybe try a few more

(Review request: http://reviews.vapour.ws/r/5616/)

[reedobrien]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[reedobrien]

jujubot failed to merge in http://juju-ci.vapour.ws:8080/job/github-merge-juju/9120/console

Trying again