When creating a virtualenv with Python 3.6.8 using the
--system-site-packages option and pip>=19.0.0 a bug affects
the virtualenv breaking pep build isolation.
This patch downgrade pip to version 19.0.0 as a
workaround.
Also converts pip3 command to pip.
See [1] for more info.
[1] https://github.com/pypa/pip/issues/6264
Change-Id: I738d7c2e50914a773920e5381305e8c5131ccd5f