model-config authorized-keys allows you to set but not retrieve config

The commands `juju ssh-keys` and add-ssh-key and remove-ssh-key all actually modify an `authorized-keys` attribute of the model config.
However, to hide that fact, when you do `juju model-config` it does not return the authorized-keys content.

In doing so, though, we made the mistake of still allowing you to do:
  juju model-config -m controller authorized-key=abcd

So you can *set* authorized-keys to something that will break your machine, but you cannot read that config.

$ juju model-config -m controller authorized-keys
ERROR "authorized-keys" is not a key of the currently targeted model: "controller"

$ juju model-config -m controller authorized-keys=abcd
WARNING key "authorized-keys" is not defined in the current model configuration: possible misspelling

juju:PRIMARY> x = db.settings.find({"_id": "1048c95a-c1b1-45f9-83e2-3abde69b54df:e"}, {"settings.authorized-keys": 1})
{ "_id" : "1048c95a-c1b1-45f9-83e2-3abde69b54df:e", "settings" : { "authorized-keys" : "abcd" } }

At the very least, if we are going to hide it from model-config, then we should refuse to let you set it.