Manage notification handler for Sensu

This patch adds two things:
  - the field 'handler' to an alarm. Now an alarm can be described as
        alarm_name:
            policy: name_of_policy
            alerting: enabled_with_notifications
            handler: mail
            ...
  - a new parameter 'notification_handler' to pass the handler that
    can now be added to an alarm.

Change-Id: I1e0767b3c1aa664110c97ae5f8fafc47c9682ab9
9 files changed