Templates for Slack notifications

The current Slack notifier posts a large JSON blob to the configured Slack channel when an alarm triggers. For example:

{
  "metrics":[
      {
        "dimensions":{
            "hostname":"old-cranky",
            "component":"nova-conductor"
         },
         "id":null,
         "name":"log.error_message"
      }
   ],
   "alarm_id":"3eddc74b-ee70-4df2-ac99-aaac093fd85f",
   "state":"ALARM",
   "alarm_timestamp":1509355709,
   "tenant_id":"a7c3c7856c224b76956ca4e246f0ae43",
   "old_state":"OK",
   "alarm_description":"",
   "message":"Thresholds were exceeded for the sub-alarms: count(log.error_message, deterministic) > 0.0 with the values: [1.0]",
   "alarm_definition_id":"71b4dbd7-7870-44f7-8366-e0aca4c3689e",
   "alarm_name":"log_error_message"
}

Although comprehensive, for some users this is overly verbose. Here we propose to add an optional template for Slack notifications which allows users to define the information included in the notification. For example, the above notification text may become:

Alarm 'log_error_message' has triggered on host 'old-cranky' in component 'nova-conductor'.