Support templating in environment variables

[rdsubhas]

Support environment variable expansion using templates /cc @vpillac

Few problems:

• Environment variables are used for parameter substitution, but if they themselves have parameters, then 2-pass/n-pass
• Ordering of the passes, with/without nested references
• Potential recursion

[pkuczynski]

Another example I wanted to do, and I can't at the moment:

env:
  GIT_COMMIT: "$(git rev-parse --abbrev-ref HEAD)"