Use k8s ServiceAccount with a restricted set of permissions instead of default

[agrimmer]

Currently, the service uses the default Service Account with a set of high privileges inside the k8s clusters.

Use a dedicated service account with a minimum set of privileges as described in our contributing guidelines here https://github.com/keptn-sandbox/contributing/pull/2

Definition of Done:

• Service either uses the keptn-default Service Account which has no privileges at all or a new, dedicated Service Account with a minimal set of permissions