Replace the legacy backups implementation with a new one.

The new implementation is refactored to fit to the new backups approach of storing the backups in env storage (along with metadata in mongo). This is the final large patch in the backup series (the remaining ones are much smaller).

The next 2 patches in the series:
https://github.com/ericsnowcurrently/juju/pull/3
https://github.com/ericsnowcurrently/juju/pull/4
Basically, it lets you determine the path that is prepended to the filename/dirname returned from the various getters.  This was useful for tests, for building the archive, and I expect it will be helpful for dealing with an unpacked archive.