Add a backups config abstraction.

This abstraction helps address the problem of hard-coded paths for various sources from which backups pull data.  It also provides a better encapsulation of the information that drives backups.
Regarding using an interface, as I implied earlier BackupsConfig is the encapsulation of the information about juju that the backup functionality needs.  The interface exposes exactly the (currently) two necessary bits.  I agree that it doesn't do anything other than provide information.  However, how it gets that information is up to the implementation.  The implementation in config/backups.go is merely a wrapper around some static state, but that's just incidental.  Furthermore, using an interface here allows me to avoid import cycles over in state/apiserver, etc.  I was going to say it helps with testing, but it actually doesn't make much of a difference there. :)