auth token middleware is an upgrade bottleneck

Steps to reproduce:

1) Install devstack stable/essex
2) Kill all services
3) Upgrade keystone to master and start
4) Upgrade glance to master and attempt to start

The exception you see from the last step will look like this:

2012-08-29 18:27:04 TRACE glance File "/opt/stack/keystone/keystone/middleware/auth_token.py", line 123, in <module>
2012-08-29 18:27:04 TRACE glance if 'config_file' in cfg.CONF:
2012-08-29 18:27:04 TRACE glance AttributeError: 'module' object has no attribute 'CONF'

What's happening is the auth_token middleware is looking for openstack.common.cfg in nova's directory and barfing on it as CONF didn't exist in the essex codebase: https://github.com/openstack/nova/blob/stable/essex/nova/openstack/common/cfg.py

This makes the auth_token middleware a bottleneck to upgrading a system as you must upgrade all the code first before you can turn on a keystone-enabled folsom service.