Allow a Cassandra database to be scaled down

[wallrj]

/kind feature

Right now, if you scale down a Cassandra database,

• the Cassandra controller will immediately decrease the StatefulSet replica count,

• the StatefulSet controller will delete the pods with the largest index.

• If there is data which is only replicated on those pods, the data will be lost.

• Create Cassandra PilotResources #152 Create PilotResources.

• Create a Cassandra Pilot #141 Create a Cassandra Pilot and make it the entrypoint.

• RBAC policy prevents Pilot from listing Pilot resources #161 RBAC policy to allow a pilot to list and update the status of Pilot resources

• Have the pilot update the status of its PilotResource #169 have the pilot update the status of its PilotResource.

• Only scale down the cluster when Pilot status shows that the Pilot has removed its database node from the cluster.

[wallrj]

Supplanted by #285