Add workaround for keystonemiddleware/neutron memcached issue

There is an issue where keystonemiddleware connections to memcached from
neutron-server grow beyond configured values [1], eventually reaching
the maximum number of connections accepted by memcached servers. Other
services do not appear to be affected by this issue.

A workaround is to use the advanced memcached pool. Despite its
documentation claiming to only work with Python 2, it appears to work
fine on Python 3.

[1] https://bugs.launchpad.net/keystonemiddleware/+bug/1883659

Change-Id: Ifbbc2022839cbc575848d830600241c61603c80b
Closes-Bug: #1892210

ansible/roles/neutron/templates/neutron.conf.j2

@@ -115,6 +115,9 @@ memcache_secret_key = {{ memcache_secret_key }}
 
 memcached_servers = {% for host in groups['memcached'] %}{{ 'api' | kolla_address(host) | put_address_in_context('memcache') }}:{{ memcached_port }}{% if not loop.last %},{% endif %}{% endfor %}
 
+# Workaround for https://bugs.launchpad.net/keystonemiddleware/+bug/1883659
+memcache_use_advanced_pool = True
+
 [oslo_messaging_notifications]
 transport_url = {{ notify_transport_url }}
 {% if neutron_enabled_notification_topics %}

releasenotes/notes/fix-neutron-memcached-connection-issue-84f5affa217b4612.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixes `LP#1892210
+    <https://bugs.launchpad.net/kolla-ansible/+bug/1892210>`__ where the number
+    of open connections to Memcached from ``neutron-server`` would grow over
+    time until reaching the maximum set by ``memcached_connection_limit`` (5000
+    by default), at which point the Memcached instance would stop working.