add stevedore for testing with Keystoneclient master

stable/* tests run with Keystoneclient master and stevedore
dependency was introduced by "Plugin loading from config objects"
https://github.com/openstack/python-keystoneclient/commit/5c91ede44768ebbb2fff12f9a7c93e63b9bbd56d

Change-Id: If70a4114d73c71a1304cce6e937b0048213e730f