add ability to pass ENV vars

[openPablo]

Description of feature

Add the ability to pass environment variables through stackl parameters

Purpose of this new feature

Pass these env variables to container images.

Additional information

[mieel]

A use case would be when we are already using a central config manager.

We could then define a environment agnostic stack config:

...
params:
  domain: ${Product.App.Common.Domain}
  central_logger: ${Operations.Logger}