feat(neutron): add no-op trunk driver for understack

This adds an effectively no-op trunk driver for the understack mechanism
driver. This allows the trunk plugin to load and work with the
undercloud ML2 mechanism. No operations are being performed here so it's
just a stub.

python/neutron-understack/neutron_understack/neutron_understack_mech.py

@@ -14,6 +14,7 @@
 
 from neutron_understack import config
 from neutron_understack.nautobot import Nautobot
+from neutron_understack.trunk import UnderStackTrunkDriver
 from neutron_understack.undersync import Undersync
 
 LOG = logging.getLogger(__name__)
@@ -110,6 +111,7 @@ def initialize(self):
         conf = cfg.CONF.ml2_understack
         self.nb = Nautobot(conf.nb_url, conf.nb_token)
         self.undersync = Undersync(conf.undersync_token, conf.undersync_url)
+        self.trunk_driver = UnderStackTrunkDriver.create(self)
 
     def create_network_precommit(self, context):
         log_call("create_network_precommit", context)

python/neutron-understack/neutron_understack/trunk.py

@@ -0,0 +1,28 @@
+from neutron.services.trunk.drivers import base as trunk_base
+from neutron_lib.api.definitions import portbindings
+from neutron_lib.services.trunk import constants as trunk_consts
+from oslo_config import cfg
+
+SUPPORTED_INTERFACES = (portbindings.VIF_TYPE_OTHER,)
+
+SUPPORTED_SEGMENTATION_TYPES = (trunk_consts.SEGMENTATION_TYPE_VLAN,)
+
+
+class UnderStackTrunkDriver(trunk_base.DriverBase):
+    @property
+    def is_loaded(self):
+        try:
+            return "understack" in cfg.CONF.ml2.mechanism_drivers
+        except cfg.NoSuchOptError:
+            return False
+
+    @classmethod
+    def create(cls, plugin_driver):
+        cls.plugin_driver = plugin_driver
+        return cls(
+            "understack",
+            SUPPORTED_INTERFACES,
+            SUPPORTED_SEGMENTATION_TYPES,
+            None,
+            can_trunk_bound_port=True,
+        )