Ability to test basic scenarios with envtests

[burmanm]

What is missing?

There's no way to run CassandraDatacenter controller in the cass-operator inside envtests.

This ticket probably existed inside some other ticket, but has since been closed.

Why is this needed?

Finding out if the operator works at all through the e2e tests is a long wait. We need to have some basic smoke tests available in the envtesting form to speed up the feedback loop when making changes.