Merge pull request #4 from relusek/master

In mongodb 2.6.10, when oplog appears in mongodb.conf, service doesnt start up
diff --git a/mongodb/files/mongodb.conf b/mongodb/files/mongodb.conf
index c8e1587..c5ac17c 100644
--- a/mongodb/files/mongodb.conf
+++ b/mongodb/files/mongodb.conf
@@ -51,7 +51,9 @@
 #   2=R
 #   3=both
 #   7=W+some reads
-oplog = {{ server.logging.get('oplogLevel', 0) }}
+{%- if server.logging.oplogLevel is defined %}
+oplog = {{ server.logging.get('oplogLevel') }}
+{%- endif %}
 
 # Diagnostic/debugging option
 #nocursors = true