[RFC] Allow MaaS alarms to be disabled

Implements functionality to disable alarms rather than disabling the
entire check. This comes from a discussion in rcbops/u-suk-dev#948
where some of the checks are good to have, but the alarms aren't
needed.

Connects rcbops/u-suk-dev#1019

rpcd/playbooks/roles/rpc_maas/defaults/main.yml

@@ -430,11 +430,20 @@ maas_pip_dependencies:
 maas_plugin_dir: /opt/rpc-openstack/maas/plugins/
 
 maas_rpc_scripts_dir: /opt/rpc-openstack/scripts
+
+# The following two variables control which checks and alarms are excluded from
+# MaaS. Here is explanation of what each variable does:
+#
+# 1) When you add a check to `maas_excluded_checks`, the check (and any
+#    associated alarms) are not provisioned. This disables graphing,
+#    monitoring, alarms and alerts for that particular check.
 #
-# maas_excluded_checks: List of checks and alarms to exclude from this deploy
+# 2) When you add an alarm to `maas_excluded_alarms`, the alarm is not
+#    provisioned. However, the check itself (and any associated graphing) is
+#    maintained.
 #
 maas_excluded_checks: []
-
+maas_excluded_alarms: []
 
 #
 # openrc definitions from OSA

rpcd/playbooks/roles/rpc_maas/templates/cpu_check.yaml.j2

@@ -4,6 +4,7 @@ disabled          : false
 period            : "{{ maas_check_period }}"
 timeout           : "{{ maas_check_timeout }}"
 alarms            :
+{% if 'idle_percent_average' not in maas_excluded_alarms %}
     idle_percent_average        :
         label                   : idle_percent_average--{{ inventory_hostname|quote }}
         notification_plan_id    : "{{ maas_notification_plan }}"
@@ -12,3 +13,4 @@ alarms            :
             if (metric["idle_percent_average"] <= {{ cpu_idle_percent_avg_threshold }}) {
                 return new AlarmStatus(WARNING, "CPU time spent idle has dropped to <= {{ cpu_idle_percent_avg_threshold }}%");
             }
+{% endif %}