Fixes lp#1616305: image metadata collection must be global.

[anastasiamac]

Currently, image metadata collection is model-bound which means that we can only deploy to a "controller" model using custom images.
This collection needs to be controller-wide to ensure that any model can find this metadata.

QA steps:

1. Bootstrap private cloud using --metadata-source
2. Deploy an application to a non-controller model either the "default" or add new one)
3. Watch status - the machine should come up using an image from metadata-source

QA for multi-region cloud:

1. Bootstrap cloud using --metadata-source that has images metadata for all regions under test
2. Deploy an application to "default" model
3. Add a new model to a different region (add-model --region=xxxx)
4. Deploy an application to this new model
5. Observe all machines come up

[axw]

LGTM, please ask wallyworld to double-check.

[anastasiamac]

$$merge$$

[jujubot]

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