support for ecdsa ssh key.

- add ssh key support (ecdsa)
- update validator.py
- modify help.py balloon and add warning

Regards!
I think config.get('ssh_keysupport') will either be a list or the string "['rsa', 'ssh-rsa', 'ecdsa', 'ecdsa-sha2']"

If it's a list, you won't want to split on ",".  (You'll probably get method not found)  If it's a string, you'll end up tih several characters you don't want: []'