Generate and display ssh public key fingerprints instead of key contents

Trello Card: https://trello.com/c/iB213gUS/8-show-individual-ssh-key-fingerprints-in-the-console

Updated files:
modified:   console/app/models/key.rb
modified:   console/app/views/settings/_keys.html.haml

Updated to generate ssh key fingerprint in the model, and display that fingerprint in the view instead of the content of the key
We support kerberos principals too which should be shown as-is and not fingerprinted.