tools tarballs now live in a directory named after their stream

Previously, tools tarballs used to all be put into a directory called "releases".
The QA team want tools tarballs instead to be located in directories named after their stream eg proposed, testing, released etc. The paths to the tarballs are in the index.json files, so existing metadata will work still.

Newly generated metadata using the generate-tools plugin will now generate the new paths.

These changes are largely mechanical - many test methods now take a stream parameter so the correct tools path can be used.

Fixes: https://bugs.launchpad.net/juju-core/+bug/1384979