helm deployment: Allow helm users two switch among production, CI, and development images.

# 🚀 Feature Request

### Relevant Package [**REQUIRED**]
EdgeX helm chart

### Description [**REQUIRED**]
In order to support add-on features such as NATS that are not compiled into EdgeX by default,
it is necessary to allow the helm chart to use the EdgeX "dev" (locally-built) images.

Provide a configuration option that allows developers to switch among the following images types:
- Released images
- EdgeX CI "tip" images
- Development images

Perhaps this might be as simple as providing pre-packaged values.yml fragments to choose different image sources.