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
I like it because it makes it clear what the cleanup actions are (they aren't buried in the syntax).  I also find it easier to read overall.  Furthermore it makes adding new cleanup actions really easy and less error prone.  However, at this scale it doesn't make much difference and I don't anticipate adding more cleanup actions. :)  I'll unroll it.