Make separate liveness/readiness probes possible

[toffer]

Feature Request

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

I'd like to make it possible to have different scripts for the liveness and readiness probes. As of #234, the LivenessProbe command in the cassandra container PodTemplateSpec is hard-coded to point to /etc/readiness-probe.sh, so there's no way to have separate scripts.

Describe the solution you'd like to see

If we add liveness-probe.sh to the bootstrap Docker image, but make it a symlink to readiness-probe.sh, then we can keep the current behavior, where we use the same script for both probes. And, this also makes it possible to override this behavior and define a separate script for each probe.

Describe alternatives you've considered

Rather than use a symlink, we could cut/paste the contents of readiness-probe.sh into liveness-probe.sh. But, using the symlink makes it more obvious that we are using the same script for both probes (in the default case).