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

>     (1) Write key to <tempfile
>     (2) Check the result of: ssh-keygen -l -f <tempfile>

I like this idea, but I'm not sure that's going to reproduce the right
behaviour. The conch code is useful because it does check that the key
is sane *and* it's a public key. The last bit prevents users from saving
private keys, which would be a bit of a security disaster, and wouldn't
work for MAAS's purposes anyway. Even the hacky approach in this branch
can distinguish between public and private keys.

If we can figure out a way to get an answer to "is this a valid looking
public key" out of ssh-keygen or another OpenSSH program (library?) then
I'm in full agreement that we should use it.

Perhaps the way to do it is to first check, in Python, that the key
starts with {ssh-{rsa,dsa,ed25519},ecdsa-sha2-nistp{256,384,521}} and
then run it through ssh-keygen -l as well.