Fix list_mappings deprecation warning message

Without this patch, the identity:list_mappings policy uses the
deprecated name and check string for the old identity:get_mapping rule,
which means the resulting OR'd policy is wrong and the deprecation
message is confusing. Use the right one.

Change-Id: I3fc1cff61df6d64ea1fc98f54f98aba0cd35145c

keystone/common/policies/mapping.py

@@ -96,7 +96,7 @@ mapping_policies = [
                 'method': 'HEAD'
             }
         ],
-        deprecated_rule=deprecated_get_mapping,
+        deprecated_rule=deprecated_list_mappings,
         deprecated_reason=DEPRECATED_REASON,
         deprecated_since=versionutils.deprecated.STEIN
     ),