Fix: Use pyenv for PyPI verify jobs

PyPI verify jobs requires Python 3.x. The tox run picks up default version
of python instead of the version made available through pyenv.

To fix this Re-factor lf-activate-venv() to skip a return, while the venv is
re-used, so that the PATH can be set.

Update the tox install and run script to Call lf-avtivate-venv().

Issue-ID: https://jira.linuxfoundation.org/browse/RELENG-4468
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>
Change-Id: Ibde3ba8beb5be75fa69c9ee6cf36a80768a8f368