cli: Use unique subjects in cockroach cert create-ca

[bdarnell]

Is your feature request related to a problem? Please describe.

cockroach cert create-ca uses a hard-coded subject of O=Cockroach,CN=Cockroach CA for all CA certificates. If the ca.crt file contains the wrong CA certificate but the subject is correct, the Go crypto libraries produce a cryptic error message crypto/rsa: verification error. If the subjects didn't match, we'd get a (slightly) better error message of x509: certificate signed by unknown authority.

Describe the solution you'd like

All CA certificates generated by cockroach cert commands should have unique subjects, probably by adding a UID field to the subject containing a UUID.

Describe alternatives you've considered
It would also be nice to allow and encourage people to set their own subject instead of just using O=Cockroach,CN=Cockroach CA,UID=234543... all the time.

Jira issue: CRDB-37173