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: I4cfe2e4c86474ec9bf69a3c2007c0277288ea2b6
Merge branch 'master' of https://github.com/openstack-dev/devstack