install_venv: pass the --upgrade argument to pip install
install_venv: pass the --upgrade argument to pip install

Subsequent runs of install_venv.sh should upgrade packages to the
latest available.

Change-Id: Ib9c9ee270b778363900d92398f00c1cab3508578

zancas/hacking,emonty/oslo-hacking,openstack-dev/hacking,zancas/hacking,hyakuhei/cleantox,hyakuhei/cleantox,emonty/oslo-hacking,openstack-dev/hacking