Check for known-bad SSH hostkeys

[fullspectrumdev]

Related to #35

Once SSH support is available, it would be useful to have Artemis report things like known-bad SSH host keys.

For example, keys from https://github.com/rapid7/ssh-badkeys/tree/master/host, the Debian OpenSSL PRNG issue, etc.

I think this would be a pretty easy thing to add using Paramiko and a database of known-bad keys.