Change: Move environment variable definition to def.cf

This should make setting environment variables a bit more visible (being
alongside many other settings a user commonly changes). And it will
hopefully make policy framework upgrades easier as it will hopefully
decrease the number of default files a user has to touch to make their
customizations.

Ref: https://dev.cfengine.com/issues/6736
Merge pull request #355 from nickanderson/redmine/6736

Change: Move environment variable definition to def.cf