Promote HEAT_ENV

At the moment we're in a conflicted situation where certain config
values must be specified in the HEAT_ENV file, but others must be set
via env vars. This spec is intended to start a discussion around
resolving that.

Design for blueprint tripleo-juno-promote-heat-env.

Change-Id: Idb49477f4b37474aac11c680a837be17b696a18d
Merge "Promote HEAT_ENV"