Create Helm feature flag for promotion feature

[mowies]

Overall Goal

As a Keptn user, I want to have a dedicated way to promote my GitOps enable app to the next stage in my environment.

Details

To stay flexible with releases in Keptn, a feature flag should be introduced to hide the new functionality until it's fully implemented.
There should be a helm value and an environment variable for the lifecycle operator to set the feature flag.

DoD

• helm value added - disabled by default
• env var added to lifecycle operator deployment
• adapt helm tests
• adapt helm charts