Feature Request: Support external etcd service

The pull request under review to integrate the new scheduler (https://github.com/apache/openwhisk-deploy-kube/pull/729) creates a new etcd service as part of the helm deployment. Although internally some of the plumbing is there to support use of an external etcd deployment, the PR does not fully implemented it.

This issue is a feature request to add support for use of an external etcd cluster when the scheduler is enabled.