add support for setting containerConcurency

[duglin]

It would be nice to be able to set the containerConcurrency field via kn:

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: helloworld
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: ${APP_IMAGE}
          containerConcurrency: 1

See last line.

Something like --max-concurrency=1