Modify app autodiscovery annotation to app.kubernetes.io/managed-by: "keptn"

[RealAnna]

In Keptn Automatic App Discovery, we have a sample KeptnApp resource that includes the following:

apiVersion: lifecycle.keptn.sh/v1alpha2
kind: KeptnApp
metadata:
  name: podtato-head
  namespace: podtato-kubectl
  annotations:
    app.kubernetes.io/managed-by: "klt"

This is because the app discovery controller hardcodes a klt annotation if the app is autogenerated. This needs to change to "keptn" but we also need to keep for a while compatibility with old CR using "klt".

AC

• Add an other annotion value set as 'keptn'
• Make sure that both annotations are checked in the create function
• When saving a new app CR only use the new annotation app.kubernetes.io/managed-by: "keptn"
• When updating an app cr that has the old annotation replace it with app.kubernetes.io/managed-by: "keptn"
• there is a separate issue to update all documentation files: remove user-visible klt strings from helm chart #2015