Make it much easier to transport SSH keys to other servers

By only having a fact on the key, we enable the use of plain resource export / collection:

    @@ssh_authorized_key{"cache-fetcher@deploy@${hostname}":
      name => "cache-fetcher@deploy@${hostname}",
      type => rsa,
      key  => sshpubkey_deploy,
      tag  => [ 'cache-fetcher' ],
    }