Flush tenant membership deletion before user

Ensure user tenant membership is *actually* removed before deleting
user.

Applied to 'stable/essex', originally committed to trunk via
https://review.openstack.org/#/c/7353/

Fixes bug 998137.

Change-Id: Ib52970d68f288b8742c3e060c7040838a1c738c2

keystone/identity/backends/sql.py

@@ -335,6 +335,7 @@ class Identity(sql.Base, identity.Driver):
             if membership_refs:
                 for membership_ref in membership_refs:
                     session.delete(membership_ref)
+                    session.flush()
 
             session.delete(user_ref)
             session.flush()