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.
Good catch! Should we put the code in this try block in a `while True` loop then?