In a deployment where Prometheus is enabled and
Alertmanager is disabled the task "Copying over
prometheus config file" in
'ansible/roles/prometheus/tasks/config.yml' will
fail to template the Prometheus configuration file
'ansible/roles/prometheus/templates/prometheus.yml.j2'
as the variable 'prometheus_alert_rules' does not
contain the key 'files'. This commit fixes this bug.
Change-Id: Idbe1e52dd3693a6f168d475f9230a253dae64480
Closes-Bug: #1854540
991bdc5f55