software/slapos-testing: Quickfix python -E errors

Since upgrading to zc.buildout = 3.0.1+slapos004, slapos.rebootstrap
and slapos.recipe.template tests are failing due to: these two tests
running buildout, buildout installing develop eggs, develop eggs now
being installed with pip install -e instead of setup.py develop, and
pip install -e launching sys.executable -E at some point. This fails
because the tests are run using gpython:main script, which currently
does not support -E option.

As a quick fix run these two tests with a barebone python executable
and use export PYTHONPATH to provide the needed eggs. This is not as
good as gpython:main because PYTHONPATH will be inherited by all sub
processes and not just those launched with sys.executable, including
potentially other versions of python.