python-keystoneclient vulnerable to context confusion in Keystone auth_token middleware

A context confusion vulnerability was identified in Keystone auth_token middleware (shipped in python-keystoneclient) before 0.7.0. By doing repeated requests, with sufficient load on the target system, an authenticated user may in certain situations assume another authenticated user's complete identity and multi-tenant authorizations, potentially resulting in a privilege escalation. Note that it is related to a bad interaction between eventlet and python-memcached that should be avoided if the calling process already monkey-patches "thread" to use eventlet. Only keystone middleware setups using auth_token with memcache are vulnerable.