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.
I realized that if there are two prompts: 

- first for the known_hosts check
- second for the user/password

then ClusterRunner may indefinitely hang.