Allow streams version to be parameterised so that it can be different for images and tools metadata
Merge pull request #736 from ericsnowcurrently/backups-apiclient

Add the API client side for backups.

Note that backups has its own facade, so it isn't quite as simple as adding a new method on Client.
Tweak streams versions constants
Merge pull request #745 from wallyworld/tools-simplestreams-v2

Allow streams version to be parameterised so that it can be different for tools and images

This started out as a branch to use "v2" simplestreams metadata for tools to allow different metadata for 1.20 and newer vs 1.18 and older version of Juju. The fix is to change how simplestreams paths are resolved such that the streams version is now parameterised.

A different direction was decided on, but this change is still useful because it simplifies how Fetch() is invoked, provided a cleaner implementation of path resolution, and allows for easy changes of the streams versions in the future when needed. The changes are mechanical.
Add a juju subcommand for backup.
Add the backups create subcommand.
Clean up names and doc comments.