BF: invoke virtualenv from a new directory to preclude older easy_install reading/following setup.cfg

Seems to work fine across platforms:
http://nipy.bic.berkeley.edu/builders/fail2ban-py2.7-win7_0.8/builds/0/steps/shell_1/logs/stdio

yoh@mike /cygdrive/c/buildslaves/mike-win7-64/fail2ban-py2_7-win7_master/mk_venv
$ cd ../venv/

yoh@mike /cygdrive/c/buildslaves/mike-win7-64/fail2ban-py2_7-win7_master/venv
$ ls -R
.:
Include  Lib  Scripts