Add github public key to known_hosts.

This is in effect a fix for gitolite installation when ssh key forwarding is not configured/working, as per the error I reported here https://github.com/al3x/sovereign/issues/261

It adds githubs public key to the deploy user's known_hosts file prior to trying to clone gitolite from github.

cons:
- Public key is hard coded, this will break and require a change if github updates their key (but how often does that happen?)
- I'm not sure the gitolite role is the best place to add this, I'd do it as part of setting up the deploy user account, but sovereign currently does that manually...but it's where its currently needed to get the vagrant testing box up and running without manual intervention.