While going through the dependency list (requirements*.txt files) and
performing our self tests out of virtual environments, I noticed that
some tests are run outside the virtual environments.

The reason is that, even though the virtual environment is activated
for the test session (and say, `which python` gives `/venv/bin/python`),
we have hard coded `/usr/bin/python` in most places.

According to the some discussions on the virtualenv project itself[1],
a quick solution is to revert to the also common `/usr/bin/env python`
way of pointing to the Python interpreter.

[1] - https://github.com/pypa/virtualenv/issues/124Signed-off-by: NCleber Rosa <crosa@redhat.com>