update SitePrivilegeAdmin and SliceMembershipAdmin
diff --git a/plstackapi/core/models/slice.py b/plstackapi/core/models/slice.py
index d174b2e..dda6553 100644
--- a/plstackapi/core/models/slice.py
+++ b/plstackapi/core/models/slice.py
@@ -56,3 +56,10 @@
 
     def __unicode__(self):  return u'%s %s %s' % (self.slice, self.user, self.role)
 
+    def save(self, *args, **kwds):
+        self.driver.add_user_role(self.user.user_id, self.slice.tenant_id, self.role.role_type)
+        super(SliceMembership, self).save(*args, **kwds)
+
+    def delete(self, *args, **kwds):
+        self.driver.delete_user_role(self.user.user_id, self.slice.tenant_id, self.role.role_type)
+        super(SliceMembership, self).delete(*args, **kwds)