Cockroach cluster now requires client-side certificates by default

[plaflamme]

Unless you start the nodes using --insecure switch, cockroach now requires client-side certificates to communicate with the cluster (first seen in cockroach-dev@76c6a93aa9b3)

scroach should make it easy to specify the certificates to use when connecting.

Unit tests use the --insecure flag currently, but it should be possible to generate client-side certificates using the cert client cockroach command. Needs investigation.