Enable super-on-old-class pylint check

.. and disable the check locally for two midonet and Ryu classes that
fail since the midonet/ryu libraries are unavailable in our toxenv.

Change-Id: Idca46f853e6a116e8b2c246b4c6cae159894f887

.pylintrc

@@ -25,7 +25,6 @@ disable=
  no-method-argument,
  no-self-argument,
  no-value-for-parameter,
- super-on-old-class,
 # "W" Warnings for stylistic problems or minor programming issues
  abstract-method,
  arguments-differ,

neutron/plugins/midonet/plugin.py

@@ -36,6 +36,9 @@ midonet_opts = [
 cfg.CONF.register_opts(midonet_opts, "MIDONET")
 
 
+# Derives from `object` (via at least NeutronDbPluginV2), but pylint
+# can't see that without having the midonet libraries available.
+# pylint: disable=super-on-old-class
 class MidonetPluginV2(plugin.MidonetMixin):
 
     vendor_extensions = plugin.MidonetMixin.supported_extension_aliases

neutron/plugins/ofagent/agent/ofa_neutron_agent.py

@@ -123,6 +123,9 @@ class Bridge(flows.OFAgentIntegrationBridge, ovs_lib.OVSBridge):
         self.get_datapath(retry_max)
 
 
+# RyuApp derives from `object`, but pylint can't see that without
+# having the ryu libraries available.
+# pylint: disable=super-on-old-class
 class OFANeutronAgentRyuApp(app_manager.RyuApp):
     OFP_VERSIONS = [ryu_ofp13.OFP_VERSION]