Use Pod labels for better manageability

[sfc-gh-kmakino]

This PR contains the following new capabilities:

• An agent scaler creates Joshua agents with a known tag baked in as AGENT_TAG. This prevents mismatching versions between the scaler and the agents
• At the boot time, an agent scaler requests all running agents to exit after the current ensemble. This guarantees to recreate agents in case both agents and the scaler are updated.
• Add a hidden command agent_tag in Joshua CLI. When ENABLE_DYNAMIC_AGENT_TAG argument is passed to the scaler, the scaler uses the agent tag stored in FDB.
• Agent pods will have a proper app label as joshua-agent
• Each agent writes its current ensemble in the Pod label ensemble

  $ kubectl --namespace joshua get pods -L app,ensemble
  NAME                                       READY   STATUS              RESTARTS   AGE   APP            ENSEMBLE
  agent-scaler-deployment-86c984887f-vdzpv   1/1     Running             0          45s   agent-scaler
  joshua-agent-221118030447-0-57dtx          1/1     Running             0          3s    joshua-agent   20221118-030444-kmakino-f067840ad81ea406
  joshua-agent-221118030447-1-mjtdf          1/1     Running             0          2s    joshua-agent   20221118-030444-kmakino-f067840ad81ea406
  joshua-agent-221118030447-2-647qb          1/1     Running             0          2s    joshua-agent   20221118-030444-kmakino-f067840ad81ea406
  joshua-agent-221118030447-3-clnhq          1/1     Running             0          2s    joshua-agent   20221118-030444-kmakino-f067840ad81ea406
  joshua-agent-221118030447-4-h6jgn          0/1     ContainerCreating   0          2s    joshua-agent   20221118-030444-kmakino-f067840ad81ea406

[sfc-gh-kmakino]

This solution isn't very elegant. I'll close this and rethink.