update the event_connection to connection for panko

more infomation, refer to the panko docs[1] or code[2]

[1]: https://docs.openstack.org/panko/rocky/install/manual.html
[2]: https://github.com/openstack/panko/blob/9c4677f1784ed3353c31067e6e1bf74a0c86c037/panko/storage/__init__.py#L39

Change-Id: Ic7bdf555eb262005ca24f675cd8cee09d9bc3092
Closes-Bug: #1690843

ansible/roles/panko/templates/panko.conf.j2

@@ -9,10 +9,10 @@ log_file = /var/log/kolla/panko/panko-api.log
 
 [database]
 {% if panko_database_type == "mongodb" %}
-event_connection = mongodb://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mongodb_address }}/{{ panko_database_name }}
+connection = mongodb://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mongodb_address }}/{{ panko_database_name }}
 metering_connection = mongodb://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mongodb_address }}/{{ panko_database_name }}
 {% elif panko_database_type == "mysql" %}
-event_connection = mysql+pymysql://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mysql_address }}/{{ panko_database_name }}
+connection = mysql+pymysql://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mysql_address }}/{{ panko_database_name }}
 metering_connection = mysql+pymysql://{{ panko_database_user }}:{{ panko_database_password }}@{{ panko_database_mysql_address }}/{{ panko_database_name }}
 {% endif %}