Auto-add hosts to known_hosts on git clone

Also, users can enable strict host key checking via clusterrunner.conf

Currently, we reject any host that isn't in the known_hosts file upon
git cloning for security reasons. However, because of the new security
measures (hmac digest validation on all requests + 600 permissions on
the clusterrunner.conf file) we have determined this restriction is no
longer necessary.
Merge pull request #22 from box/automatically_add_unknown_host_on_git_clone

Auto-add hosts to known_hosts on git clone