Fix: Use lf-activate-venv to install openstack dep

Fix: Use lf-activate-venv to install openstack dep

Using python-tools-install.sh for the pre/post build is not recommended
approach for installing python dependencies since this installs the
dependencies with `--user` option (removed in I821a86ac3b54f284e8).

Instead use lf-activate-venv to setup an venv and pull in the required
dependencies and save the path of the virtualenv in a temp file that
can be checked before attempting to create a venv.

Issue-ID: RELENG-4357
Change-Id: I00bbf08921a3ddc6349e74e9cd9d6316b0876749
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>