Define default value for neutron_default_availability_zones

OVN driver does have a different behaviour whenever scheduler hint
is provided or not [1]. In case it's not provided, it will filter out
all chassis which do not have AZ specified[2]. With that, we supply
an AZ for each chassis starting with [3].

So most painless way forward from here is to ensure that same hint will
be supplied for scheduling.

With that the change should be also safe for OVS/LXB drivers.

[1] e9acc14d37/neutron/common/ovn/utils.py (L1071-L1078)
[2] e9acc14d37/neutron/common/ovn/utils.py (L896-L897)
[3] https://review.opendev.org/c/openstack/openstack-ansible-os_neutron/+/904572/2/tasks/providers/setup_ovs_ovn.yml

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible-openstack_hosts/+/930272
Change-Id: I6caa3f682133c89c1c997599043fe142c84eb016

defaults/main.yml

@@ -347,15 +347,21 @@ neutron_service_in_ldap: "{{ service_ldap_backend_enabled | default(False) }}"
 ###
 ### Availability zones
 ###
-# Default availability zones do define a list of zones to where routers/agents
-# will be scheduled by default. This is a list, since a deployment might stretch
-# networks across AZs
-neutron_default_availability_zones: []
-
 # Availability zone defines current AZ of the component. For OVN you can define
 # multiple AZs separated with a colon, ie "az1:az2"
 neutron_availability_zone: nova
 
+# Default availability zones do define a list of zones to where routers/agents
+# will be scheduled by default. This is a list, since a deployment might stretch
+# networks across AZs.
+# Default: Neutron will attempt scheduling across all defined AZs for Neutron hosts.
+neutron_default_availability_zones: >-
+  {{
+    groups['neutron_all'] | map(
+      'extract', hostvars, 'neutron_availability_zone') | map(
+        'default', neutron_availability_zone) | map('split', ':') | flatten | unique
+  }}
+
 ###
 ### Telemetry integration
 ###

releasenotes/notes/neutron_default_az-51715b4d836b5802.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    Changed a default value for a ``neutron_default_availability_zones``
+    variable. From now on role will gather defined ``neutron_availability_zone``
+    across all hosts (which defaults to `nova`) and attempt to schedule
+    on all available Availability Zones by default.
+    Change of the default should not alter behaviour for single-AZ setups.