Clean up how test env variables are parsed

Oslo and cookiecutter and other projects were using
variable to parse test environment vaiables. Let us
do this in Nova

This is copied from 739627bf79619f8 Use cleaner version from
cookiecutter OpenStack template

Change-Id: I4c2b4072981985e7632fc775ffc98216fbfc0508
Merge "Clean up how test env variables are parsed"