Fix Prometheus template generation

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

ansible/roles/prometheus/templates/prometheus.yml.j2

@@ -5,7 +5,7 @@ global:
   external_labels:
     monitor: 'kolla'
 
-{% if prometheus_alert_rules is defined and prometheus_alert_rules.files | length  > 0 %}
+{% if prometheus_alert_rules.files is defined and prometheus_alert_rules.files | length  > 0 %}
 rule_files:
 {% for rule in prometheus_alert_rules.files %}
   - "/etc/prometheus/{{ rule.path | basename }}"

releasenotes/notes/prometheus-config-when-alertmanager-disabled-0090c1570ff4e632.yaml

+---
+fixes:
+  - |
+    Fixes templating of Prometheus configuration when Alertmanager is disabled.
+    In a deployment where Prometheus is enabled and Alertmanager is disabled the
+    configuration for the Prometheus will fail when templating as the variable
+    `prometheus_alert_rules` does not contain the key `files`. For details
+    please see <https://bugs.launchpad.net/kolla-ansible/+bug/1854540>`__.