Add a juju restore subcommand.

This is the CLI portion for the work we are doing on cleaning up the juju-restore plugin (and turning it into a normal command).  For now it just stubs out the actual call to the restore client.
as with Backup, we'll want to consider what we want to do when the actual API for "Restore" is not available.
Actually, given the purpose of the function, I really think we need to be doing this differently.
I don't know Nate/Horacio's final design for how "juju restore" is supposed to work. But if you look at the help text from above, it is expected to bootstrap a new instance and do a lot more work than just send some bytes to a remote server and assume that server restores itself to a healthy condition.