Enable litteral Sub and Ref in Env vars

[JohnPreston]

Is your feature request related to a problem? Please describe.
As a developer I would want to be able to set an env var and use the native CFN sub functions to substitute values in CFN.

Describe the solution you'd like

services:
  serviceA:
    environment:
      accountId: ${AWS::AccountId}"
      endpoint:  "s3.${AWS::Region}.${AWS::URLSuffix}"

In the template translates as

accountId: !Sub "${AWS::AccountId}"
endpoint: !Sub "s3.${AWS::Region}.${AWS::URLSuffix}"