Add support for inheriting Region and Endpoint from the very top level cloud metadata
Merge pull request #104 from wallyworld/simplestreams-inheritance

Add support for inheriting Region and Endpoint from top level

We never supported inheriting region and endpoint attributes from top level simplestreams metadata but it appears some users do generate files with this expectation. So this branch adds that support.

Fixes: https://bugs.launchpad.net/juju-core/+bug/1329805