Allow bootstrap.py to work without virtualenvwrapper

This will make it easier to run Bodhi's tests on Jenkins.

The basic is that we check for the existance of the file:
  /usr/bin/virtualenvwrapper.sh
from there we have a series of if/else tuning the settings to either rely
on the classic virtualenv or on virtualenvwrapper if the later is installed
Typo fix VENVWAPPER -> VENVWRAPPER

Thanks @bochecha for pointing it out
Merge pull request #80 from fedora-infra/venv

Allow bootstrap.py to work without virtualenvwrapper