Geo: Add SSH Keygen resource and create keys automatically for the git user

This is required for Geo (gitlab-org/gitlab-ee#76) installs and does not give any downside for regular EE installs. Code is based on https://supermarket.chef.io/cookbooks/ssh_keygen but without using poise dependency.

The code is multi-platform as it uses ruby binding to openssl in order to generate the certificate, instead of shelling out to ssh-keygen that @marin warned can behave very different from platform <> distro version

Closes #1680 (closed)