fix unicode triggered failure in AuthManager

* Always return a string from AuthManager._build_mc_key()
* Fixes bug 938375

Change-Id: Iffceffab8ae62830202de07e3237230dab4e5a60

Authors

@@ -146,6 +146,7 @@ Rick Clark <rick@openstack.org>
 Rick Harris <rconradharris@gmail.com>
 Rob Kost <kost@isi.edu>
 Russell Bryant <rbryant@redhat.com>
+Russell Sim <russell.sim@gmail.com>
 Ryan Lane <rlane@wikimedia.org>
 Ryan Lucio <rlucio@internap.com>
 Ryu Ishimoto <ryu@midokura.jp>

nova/auth/manager.py

@@ -403,7 +403,7 @@ class AuthManager(object):
         key_parts = ['rolecache', User.safe_id(user), str(role)]
         if project:
             key_parts.append(Project.safe_id(project))
-        return '-'.join(key_parts)
+        return utils.utf8('-'.join(key_parts))
 
     def _clear_mc_key(self, user, role, project=None):
         # NOTE(anthony): it would be better to delete the key