Refactor Zookeeper to avoid hardcoding fixed number of deployments/services

[ramhiser]

Currently, our Zookeeper implementation uses 3 fixed Deployments and 3 corresponding fixed Services.

PetSets look like a promising direction to avoid hardcoding the nodes because they offer:

• a stable hostname, available in DNS
• an ordinal index
• stable storage: linked to the ordinal & hostname

Zookeeper is even mentioned in the PetSet docs. There's even a zookeeper example in kubernetes/contrib.

[ramhiser]

On the Kubernetes blog, there's a good example of using PetSets to deploy a 1000-node Cassandra cluster.

[adrianmkng]

I'm looking into using StatefulSets for managing this, hope to create a PR for this once I've figured it out.