Merge pull request #1249 from wallyworld/generate-old-simplestreams-index

Backport from master

When writing metadata, copy any released metadata to the legacy index file

Fixes: https://bugs.launchpad.net/juju-core/+bug/1396981

The legacy index.json file needs to be written out to contain any released metadata. This allows the current release scripts to work.

(Review request: http://reviews.vapour.ws/r/560/)

(Review request: http://reviews.vapour.ws/r/570/)