add support for availability zone in OVN cms options

LP#2002449 That fix enables AZ support in OVN via external-ids:ovn-cms-options
It relies on JUJU_AVAILABILITY_ZONE env variable.

config.yaml

@@ -297,6 +297,16 @@ options:
       .
       NOTE: It is also possible to enable this option on several OVN chassis
       applications at the same time, e.g. on 2 out of 3.
+  customize-failure-domain:
+    type: boolean
+    default: false
+    description: |
+      Juju propagates availability zone information to charms from the
+      underlying machine provider such as MAAS and this option allows the
+      charm to use JUJU_AVAILABILITY_ZONE to set default_availability_zone for
+      Nova nodes. This option overrides the default-availability-zone charm
+      config setting only when the Juju provider sets JUJU_AVAILABILITY_ZONE.
+      More details: https://docs.openstack.org/neutron/latest/admin/ovn/availability_zones.html
   nagios_context:
     default: "juju"
     type: string

lib/charms/ovn_charm.py

@@ -1341,6 +1341,10 @@ def _get_ovn_cms_options(self):
         if self.options.card_serial_number:
             cms_opts.append(
                 f'card-serial-number={self.options.card_serial_number}')
+        if self.options.customize_failure_domain and \
+           'JUJU_AVAILABILITY_ZONE' in os.environ:
+            cms_opts.append(
+                f'availability-zones={os.environ["JUJU_AVAILABILITY_ZONE"]}')
         return cms_opts
 
     def render_nrpe(self):

unit_tests/test_lib_charms_ovn_charm.py

@@ -15,6 +15,7 @@
 import collections
 import copy
 import io
+import os
 import textwrap
 import subprocess
 import unittest.mock as mock
@@ -764,6 +765,7 @@ def setUp(self):
             'vpd-device-spec': '',
             'pmd-cpu-set': '',
             'ovn-source': 'distro',
+            'customize-failure-domain': False,
         })
         self.patch_object(ovn_charm.OVNConfigurationAdapter,
                           '_ovs_dpdk_cpu_overlap_check')
@@ -1097,6 +1099,7 @@ def setUp(self):
             '[{"bus": "pci", "vendor_id": "beef", "device_id": "cafe"}]',
             'ovn-source': 'distro',
             'ovs-exporter-channel': '',
+            'customize-failure-domain': False,
         })
 
     def test_optional_openstack_metadata(self):
@@ -1734,6 +1737,13 @@ def test_assess_exporter_no_channel_not_installed(self):
         self.refresh.assert_not_called()
         self.remove.assert_not_called()
 
+    @mock.patch.dict(os.environ, {"JUJU_AVAILABILITY_ZONE": "az1"}, clear=True)
+    def test_configure_customize_failure_domain(self):
+        self.target.options.customize_failure_domain = True
+        self.assertEqual(
+            self.target._get_ovn_cms_options(),
+            ['enable-chassis-as-gw', 'availability-zones=az1'])
+
 
 class TestOVNChassisCharmOvsExporter(Helper):