Support multiple replicas of job-executor-service

[christian-kreuzberger-dtx]

In order to scale up job-executor-service using multiple replicas, we need to define the following on the keptn/distributor:;

            - name: PUBSUB_GROUP
              valueFrom:
                fieldRef:
                  fieldPath: metadata.labels['app.kubernetes.io/name']

When scaling up job-executor-service deployment, multiple instances of keptn/distributor and job-executor-service will run, but only one replica will receive the cloudevent.

Definition of Done

• Multiple replicas of job-executor-service can run
• Only one replica will receive one CloudEvent
• Multiple CloudEvents are split across multiple replicas