Use a different virtualenv for each backdrop app

If backdrop_read, backdrop_write, backdrop_admin use the same virtualenv
(~/.virtualenvs/backdrop) there's a race condition the first time ``bowl
backdrop`` is called (when no virtualenv exists) as they all try to run
virtualenv and pip install at the same time.

This commit gives them all unique virtualenv directories.
Merge pull request #190 from alphagov/fix-race-condition

Use a different virtualenv for each backdrop app