Rename no_alerting field

This patch uses a boolean to enable or disable alerting instead of using
a variable no_alerting that is not very obvious and adds unneeded
complexity.

Change-Id: I42feeb2858aaadd9c1b4aaf5217dfe34aae35059
diff --git a/_modules/heka_alarming.py b/_modules/heka_alarming.py
index efbd091..83a6445 100644
--- a/_modules/heka_alarming.py
+++ b/_modules/heka_alarming.py
@@ -4,7 +4,7 @@
 
 _valid_dimension_re = re.compile(r'^[a-z0-9_/-]+$')
 _disallowed_dimensions = ('name', 'value', 'hostname', 'member',
-                          'no_alerting', 'tag_fields')
+                          'alerting_enabled', 'tag_fields')
 
 
 def alarm_message_matcher(alarm, triggers):