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
diff --git a/_modules/heka_alarming.py b/_modules/heka_alarming.py
index 83a6445..8376e84 100644
--- a/_modules/heka_alarming.py
+++ b/_modules/heka_alarming.py
@@ -4,7 +4,8 @@
 
 _valid_dimension_re = re.compile(r'^[a-z0-9_/-]+$')
 _disallowed_dimensions = ('name', 'value', 'hostname', 'member',
-                          'alerting_enabled', 'tag_fields')
+                          'alerting_enabled', 'notification_enabled',
+                          'notification_handler', 'tag_fields')
 
 
 def alarm_message_matcher(alarm, triggers):