add support for credentials as env variables for individual docker images

LLM modules that use a commercial LLM need credentials (e.g. API Key) to use the LLM service. There should be a way to give these confidential credentials to the docker conatiner at startup-time as a environment variable.

• extend the playground config.json to contain the docker image descriptor and associated environment variable and value
• extend the kubernetes-client-script.py to add the env variable to the correct deployment.yaml