Add support for using index2.json for tools

Fixes: https://bugs.launchpad.net/juju-core/+bug/1389807

With this change, when loading tools metadata, firstly we look for an index2.json file.
If that's not there, we look for index.json

This allows extra metadata to be added for use by 1.21 and later versions of Juju, and 1.20 and ealrier will not be affected.

The metadata generation plugins have been changed to generate index2.json

As a drive by, some old code, no user used since provider storage was dropped, was deleted.