Unable to use non-dockerhub Cassandra images due to dependence on the Dockerhub image entry point script

[wallrj]

The Cassandra Pilot is currently hard coded to execute the Dockerhub image entry point script.

https://github.com/docker-library/cassandra/blob/master/3.11/docker-entrypoint.sh

This allows us to use environment variables to control the configuration of the Cassandra process.

But it also locks us to using that image.

Instead, we might bundle our own default Cassandra configuration file and execute cassandra directly.

In future we might provide a mechanism for merging the default configuration file, supplied in the image, with the configuration variables that we need to override for Navigator to manage the cluster.

/kind bug