Make backup IDs more human-readable.

Currently we just use the current UUID.  This patch changes it to "<timestamp>.<env ID>".  A more human-readable ID will help juju users.
I'd appreciate a doc comment describing the chosen format. Assuming this is for the benefit of the user, why not make this also user-configurable? It could be an environment config setting (e.g. "backup-filename") with a simple format, i.e. a string with tokens like %X, where X is Y, m, d, H, M, S, all with fixed representation, maybe including other relevant values, like %host, %machine, %env, %ver, %checksum, etc.) ? I don't want to block this PR because of my suggestion, but it could be done later in a follow-up, if it's worth giving the user that flexibility.