kibana: add configuration option for base url

Recent versions of ELK complain when this URL is not
set. This parameters allows users to set the URL which
is used to access Kibana.

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible-ops/+/843423

Change-Id: I4cafc29bde353f4cf1286af73617b1ad3d5fea3d

elk_metrics_7x/roles/elastic_kibana/defaults/main.yml

@@ -36,3 +36,6 @@ kibana_elastic_endpoints:
 # kibana_reporting_encryption_key: ""
 # The key used to encrypt kibana session cookies
 # kibana_security_encryption_key: ""
+
+# The URL which users access Kibana from
+# kibana_base_url: ""

elk_metrics_7x/roles/elastic_kibana/templates/kibana.yml.j2

@@ -18,6 +18,12 @@ server.host: {{ kibana_interface }}
 # default to `true` starting in Kibana 7.0.
 #server.rewriteBasePath: false
 
+# Specifies the public URL at which Kibana is available for end users. If
+# `server.basePath` is configured this URL should end with the same basePath.
+{% if kibana_base_url is defined %}
+server.publicBaseUrl: "{{ kibana_base_url }}"
+{% endif %}
+
 # The maximum payload size in bytes for incoming server requests.
 #server.maxPayloadBytes: 1048576