Allow comma-separated list on event filters for distributors

[johannes-b]

Use-Case:

For implementing the above use-case, I need two helm-services: (1) works on control-plane and deploys on development, hardening, production; (2) works on execution-plane and deploys on production-remote

Limitation: Filters on the distributor just allow a single value.

Improvement: As a user, I would like to configure my distributor to listen to events, e.g., for multiple stages. In the example below, I would like to get events for the dev and hardening stage:

distributor:
  projectFilter: ""                 
  stageFilter: "development,hardening,production"             
  serviceFilter: ""                     

Implementation details:

• This should work on project/stage/service-level
• The corresponding function is here: https://github.com/keptn/keptn/blob/master/distributor/cmd/main.go#L681
• Instead of doing a equals comparison, a contains could address this limitation

Definition of Done:

• When setting a filter, a distributor can process events for multiple stags/projects/services
• Update of Readme.md and keptn/docs (https://keptn.sh/docs/0.8.x/operate/multi_cluster/#how-to-deploy-an-execution-plane-services)