Take rule value field into account

An alarm rule may include a field "value" specifying the name of
the value field to use. That field was not taken into account.
diff --git a/heka/files/lma_alarm.lua b/heka/files/lma_alarm.lua
index b826112..56530d3 100644
--- a/heka/files/lma_alarm.lua
+++ b/heka/files/lma_alarm.lua
@@ -35,6 +35,9 @@
             {%- endfor %}
           },
           {%- endif %}
+          {%- if _rule.value is defined %}
+          ['value'] = '{{ _rule.value }}',
+          {%- endif %}
         },
         {%- endfor %}
       },