Code review comment for lp:~allenap/maas/other-ssh-key-types--bug-1590081

> Oh, and I guess you could run the key through "ssh-keygen -l -f
> <keyfile>" first to grab the comment. You'd have to parse the output;
> it seems to append the key type in parentheses after the comment, and
> spaces are allowed in the comment. So you'd just drop the first,
> second, and last fields, and keep everything in the middle.

There's no official documentation for the output of `ssh-keygen -l` so
I'd rather parse the given key because we do have some kind of
documentation for what that ought to look like.