Add -U to pip install command in tox.ini
Add -U to pip install command in tox.ini

Ask pip to upgrade packages in the virtualenv so that a long-lived
virtualenv is still kept up to date as external dependencies change.

Change-Id: Ife76a83be95fed7cc4b99bb50a97277d0751e76c

faizan-barmawer/elytics,citrix-openstack-build/ironic-lib,faizan-barmawer/ironic-lib,openstack/ironic-lib,NaohiroTamura/ironic-lib