Support empty *_monitored_queues for environments without telemetry

[jaison]

OSP16 environments without this queue or without telemetry will face error/traceback in collectd.
Although adding non-existent queues does not functionaly break collectd, but collectd will use its script/pyrabbit to connect to rabbitmq every 10 seconds.
We should support empty values like below so that no queue message counts are seen for any queues, but at the same time other results/metrics from pyrabbit are reported.

[jaison (jaison1984)]

./browbeat-containers/collectd-openstack/files/collectd_rabbitmq_monitoring.py supports having no message_count.
Need to make sure such collectd file should add message_count config only when *_monitored_queues is not empty

./ansible/install/roles/collectd/templates/undercloud.collectd.conf.j2