PetSets for named services/agents

OpenStack has several rpc based services/agents that register themselves in databases (such as heat-engine, nova-conductor). In most cases, they don't really need to be run on any specific host, only that there's a service/agent with that name running somewhere.

PetSets (http://kubernetes.io/docs/user-guide/petset/) where built specifically for this case. They let containers have a consistent name, and optionally a persistent volume for state. They still allow the pod to run anywhere like a RC and scale up/down.

We should be able to do something like the following:
apiVersion: v1
kind: Service
metadata:
  name: heat-engine
  labels:
    app: heat-engine
spec:
  ports: []
  clusterIP: None
  selector:
    app: heat-engine
---
apiVersion: apps/v1alpha1
kind: PetSet
metadata:
  name: heat-engine
spec:
  serviceName: "heat-engine"
  replicas: 3
  template:
    metadata:
      labels:
        app: heat-engine
      annotations:
        pod.alpha.kubernetes.io/initialized: "true"
    spec:
      containers:
      - name: heat-engine
        image: ...heat-engine....
      ...