Disable dns-integration API extension if it's not enabled in ML2

In case when dns-integration ML2 extension driver isn't enabled,
ovn_l 3 service plugin will now not load "dns-integration"
API extension to list it as supported.

This patch also adds method supported_extension_aliases() which was
originally introduced in patch [1] but it can't be backported.

Conflicts:
    neutron/services/ovn_l3/plugin.py

[1] https://review.opendev.org/c/openstack/neutron/+/722415

Closes-bug: #1906311
Change-Id: I6a7ff14057d80196feb5fd9d47a6a73b29bc768d
(cherry picked from commit e70e3ab455)

neutron/services/ovn_l3/plugin.py

@@ -12,9 +12,11 @@
 #    under the License.
 #
 
+from neutron_lib.api.definitions import dns as dns_apidef
 from neutron_lib.api.definitions import external_net
 from neutron_lib.api.definitions import portbindings
 from neutron_lib.api.definitions import provider_net as pnet
+from neutron_lib.api import extensions as api_extensions
 from neutron_lib.callbacks import events
 from neutron_lib.callbacks import registry
 from neutron_lib.callbacks import resources
@@ -63,7 +65,7 @@ class OVNL3RouterPlugin(service_base.ServicePluginBase,
 
     # TODO(mjozefcz): Start consuming it from neutron-lib
     # once available.
-    supported_extension_aliases = (
+    _supported_extension_aliases = (
         extensions.ML2_SUPPORTED_API_EXTENSIONS_OVN_L3)
 
     @resource_registry.tracked_resources(router=l3_models.Router,
@@ -85,6 +87,20 @@ class OVNL3RouterPlugin(service_base.ServicePluginBase,
             self.create_floatingip_precommit, resources.FLOATING_IP,
             events.PRECOMMIT_CREATE)
 
+    @staticmethod
+    def disable_dns_extension_by_extension_drivers(aliases):
+        core_plugin = directory.get_plugin()
+        if not api_extensions.is_extension_supported(
+                core_plugin, dns_apidef.ALIAS):
+            aliases.remove(dns_apidef.ALIAS)
+
+    @property
+    def supported_extension_aliases(self):
+        if not hasattr(self, '_aliases'):
+            self._aliases = self._supported_extension_aliases[:]
+            self.disable_dns_extension_by_extension_drivers(self._aliases)
+        return self._aliases
+
     @property
     def _ovn_client(self):
         if self._ovn_client_inst is None: