Auto-add hosts to known_hosts on git clone

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.
Not that it matters much, but do you think we should we make these URLs un-Box-specific?