ml2 ovs: Deprecate unused [agent] veth_mtu

The [agent] veth_mtu parameter has had unused since the [ovs]
use_veth_interconnection parameter was removed by [1] during Wallaby.

[1] https://review.opendev.org/c/openstack/neutron/+/759947

This change formally deprecate the parameter so that we can remove it
in a next cycle.

Change-Id: Ib85959fbc06928a49df7ea104eae3aca3f04e091
Closes-Bug: #1957180

neutron/conf/plugins/ml2/drivers/ovs_conf.py

@@ -188,6 +188,10 @@ agent_opts = [
     cfg.PortOpt('vxlan_udp_port', default=n_const.VXLAN_UDP_PORT,
                 help=_("The UDP port to use for VXLAN tunnels.")),
     cfg.IntOpt('veth_mtu', default=9000,
+               deprecated_for_removal=True,
+               deprecated_since="Yoga",
+               deprecated_reason="This parameter has had no effect since "
+                                 "the Wallaby release.",
                help=_("MTU size of veth interfaces")),
     cfg.BoolOpt('l2_population', default=False,
                 help=_("Use ML2 l2population mechanism driver to learn "

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py

@@ -169,7 +169,6 @@ class OVSNeutronAgent(l2population_rpc.L2populationRpcCallBackTunnelMixin,
                               datapath_type=ovs_conf.datapath_type)
             for b in ('br_int', 'br_phys', 'br_tun'))
 
-        self.veth_mtu = agent_conf.veth_mtu
         self.available_local_vlans = set(range(n_const.MIN_VLAN_TAG,
                                                n_const.MAX_VLAN_TAG + 1))
         self.tunnel_types = agent_conf.tunnel_types or []

neutron/tests/unit/plugins/ml2/drivers/openvswitch/agent/test_ovs_tunnel.py

@@ -66,8 +66,6 @@ class DummyVlanBinding(object):
 
 
 class TunnelTest(object):
-    VETH_MTU = None
-
     def setUp(self):
         super(TunnelTest, self).setUp()
         self.useFixture(test_vlanmanager.LocalVlanManagerFixture())
@@ -313,7 +311,6 @@ class TunnelTest(object):
         cfg.CONF.set_override('bridge_mappings', self.NET_MAPPING, 'OVS')
         cfg.CONF.set_override('polling_interval', 2, 'AGENT')
         cfg.CONF.set_override('tunnel_types', ['gre'], 'AGENT')
-        cfg.CONF.set_override('veth_mtu', self.VETH_MTU, 'AGENT')
         cfg.CONF.set_override('minimize_polling', False, 'AGENT')
         cfg.CONF.set_override('enable_ipv6', False, 'DHCP')
 

releasenotes/notes/deprecate-agent-veth_mtu-748ca450e32ea192.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    The ``[agent] veth_mtu`` parameter of ML2 OVS mechanism driver
+    configuration has been deprecated. This parameter has had no effect since
+    the Wallaby release.