Disable Alertmanager's peer gossip in non-HA deployments

Reference:

https://github.com/prometheus/alertmanager#turn-off-high-availability

Closes-Bug: #1926463
Change-Id: I60e1dedeac25fa8fe9538a3a8e582bd8cc9324d7
(cherry picked from commit b300f7b)

Author: cityofships

ansible/roles/prometheus/templates/prometheus-alertmanager.json.j2

@@ -1,5 +1,5 @@
 {
-    "command": "/opt/prometheus_alertmanager/alertmanager --config.file=/etc/prometheus/alertmanager.yml --web.listen-address={{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --web.external-url={{ internal_protocol }}://{{ kolla_internal_fqdn | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} {% if groups["prometheus-alertmanager"] | length > 1 %} --cluster.listen-address={{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_cluster_port }} {% for host in groups["prometheus-alertmanager"] %} --cluster.peer={{ 'api' | kolla_address(host) | put_address_in_context('url') }}:{{ hostvars[host]['prometheus_alertmanager_cluster_port'] }}{% endfor %}{% endif %} --storage.path /var/lib/prometheus",
+    "command": "/opt/prometheus_alertmanager/alertmanager --config.file=/etc/prometheus/alertmanager.yml --web.listen-address={{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --web.external-url={{ internal_protocol }}://{{ kolla_internal_fqdn | put_address_in_context('url') }}:{{ prometheus_alertmanager_port }} --cluster.listen-address={% if groups["prometheus-alertmanager"] | length > 1 %}{{ api_interface_address | put_address_in_context('url') }}:{{ prometheus_alertmanager_cluster_port }} {% for host in groups["prometheus-alertmanager"] %} --cluster.peer={{ 'api' | kolla_address(host) | put_address_in_context('url') }}:{{ hostvars[host]['prometheus_alertmanager_cluster_port'] }}{% endfor %}{% endif %} --storage.path /var/lib/prometheus",
     "config_files": [
         {
             "source": "{{ container_config_directory }}/prometheus-alertmanager.yml",

releasenotes/notes/disable-alertmanager-clustering-ec70f5f970c4933a.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixes potential issue with Alertmanger in non-HA deployments. In this
+    scenario, peer gossip protocol is now disabled and Alertmanager won't
+    try to form a cluster with non-existing other instances.
+    `LP#1926463 <https://bugs.launchpad.net/kolla-ansible/+bug/1926463>`__