Allow user to specify SSH key(s) to use

[thefinn93]

The current behavior appears to be to check for the presence of ~/.ssh/id_rsa and if it doesn't exist generate it, then set that as the only key that can access the CoreOS boxes. I would prefer to use a different key, so perhaps there could be a command line flag?

Also on this note, I had a single SSH key on my machine when I tried kube-linode, but it was ignored because it wasn't and RSA key. Perhaps the mechanism to detect existing keys could use something like ssh-add -L, which will include all keys.