snapcraft create-key doesn't require login, but snapcraft list-keys does

[Leo Arias]

You can call snapcraft create-key without having logged in.
The problem is that then you can see the key you have just created without logging in.

This is a weird situation. And it might make people suspicious, because why do you need the network to query for a local key.

To reproduce:
$ snapcraft logout
$ snapcraft create-key
$ snapcraft list-keys
No valid credentials found. Have you run "snapcraft login"?
Invalid credentials: Root macaroon not in the config file.

I think that the login is requested to check if the local keys have been registered in the server. If that is the case, maybe we can print that it's required to login to check the registered keys in the server.