pypi: detect virtualenv
pypi: detect virtualenv

Avoids failure on virtualenvs:

Can not perform a '--user' install. User site-packages are not visible in this virtualenv.
Couldn't install pip, setuptools, twine or wheel.

Makes pip download less verbose.

Signed-off-by: Sorin Sbarnea <8938dff14221394d430a023580a32caf3bfb26a9@redhat.com>

testfairy/dpl,travis-ci/dpl,testfairy/dpl,testfairy/dpl,travis-ci/dpl,travis-ci/dpl