DRY up the env files

This removes the duplication that existed between defaults.env and
tests/defaults.env. Now the layering is: defaults.env, tests/test.env,
tests/local.env. Paves the way for a reorg of the defaults.env file to
add more comments, etc.
Merge pull request #2242 from gittip/dry-env-files

DRY up the env files