Introduce env to configure-aspen.py

On #2226 I added a wireup._env function and used it under the hood in
wireup to replace some older code. This commit brings configure-aspen
into the mix. By introducing the env object into configure-aspen I'm
setting us up to replace os.environ calls in other wireup functions with
the env object.
Merge pull request #2227 from gittip/wireup-env

Wireup env