WIP: 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.
Hmm.  I'm thinking this test is just reproducing testing that is done on the general cmd API.  We don't need to verify that the help message is built correctly from the command's Info.  I'll look into reducing this (and not generalizing the test for all commands).