Pin version of setuptools

Latest release of setuptool 8.0 made several versions used in
requirements.txt of OpenStack projects invalid. Instances:
* SQLAlchemy>=0.8.4,<=0.8.99,>=0.9.7,<=0.9.99 in oslo.db 1.2.0
* python-neutronclient 2.3.9.40.g9ed73c0 in openstackclient

Cap '<8.0' is set as a temporary fix until a better solution
comes up.

Change-Id: I74530bd52cf21951d9b7c26133ed21f46c89c7d1