Ability to specify canaryService Service object to reach only canary pods about argo-rollouts

[jessesuen]

the controller will modify the specified service to add the pod-hash as a label selector.

Instead of adding a new pod-hash as a label selector to the specified canaryService, we probably need to replace entire label selectors to only match canary pods. The reason why we cannot add to any existing selectors, is because we additionally want to support "previewing" pods of the new spec (#92). In order to support previewing, the selectors of the canaryService would need to be:

  selector:
    rollouts-pod-template-hash: 7d95bc4d88

instead of:

  selector:
    app: guestbook
    rollouts-pod-template-hash: 7d95bc4d88

The former is needed because if we have the latter, it implies the preview replicaset would have to have app: guestbook in its pod labels. Having app: guestbook in the preview replicaset's pod labels would cause the active/production service, to direct traffic to the preview replicaset, defeating the entire purpose of a preview.

[jessesuen]

we probably need to replace entire label selectors to only match canary pods.

Since replacing label selectors is destructive (and would additionally cause an OutOfSync condition in argo-cd), I propose it is a user error when a user specifies a canaryService which has anything in spec.selector.

[jessesuen]

For this to work, we will need to inject the rollouts-pod-template-hash label to the replicasets created by the canary strategy. This is something we already do for the blue-green strategy, but will need to do for canary to accomplish this feature.

Actually looks like we already do this for canary