Python versions prior to 2.6 have an issue clearing the environment variables
using ``os.environ.clear()`` (See http://bugs.python.org/issue3227).

Instead of modifying ``os.environ`` directly we use the ``subprocess``
module to run the commands in child processes which are given an explicit
environment which is a copy of the current ``os.environ`` augmented with
the per-part overrides.

See https://github.com/hexagonit/hexagonit.recipe.cmmi/issues/issue/1/#issue/1/comment/605362
for details.

Due to this change the hook scripts no longer have the augmented environment.
They can still access the buildout configuration to read the overrides but
need to do this manually.