Enable server to manage ssh keys

Hi @example42,

This PR enables users to centrally manage SSH-keys. The first time of provisioning it generates them, and after that it will pull them from the server.

This PR does rely on https://github.com/example42/puppi/pull/67

Am interested to hear how you think about it!

Credits go to this person btw: http://jsosic.wordpress.com/2012/12/04/managing-ssh-host-keys-with-puppet/ he did all the hard work.