Mark configuration settings in NetFlowCodec as optional (#13)

While these settings are in fact *not* optional, this seems to be the only way
to make existing NetFlow inputs work with the newly added configuration settings
without deleting and re-creating the inputs.

Fixes #12

(cherry picked from commit cfbadbd)

Author: joschi

src/main/java/org/graylog/plugins/netflow/codecs/NetFlowCodec.java

@@ -27,6 +27,7 @@
 import org.graylog2.plugin.Message;
 import org.graylog2.plugin.configuration.Configuration;
 import org.graylog2.plugin.configuration.ConfigurationRequest;
+import org.graylog2.plugin.configuration.fields.ConfigurationField;
 import org.graylog2.plugin.configuration.fields.NumberField;
 import org.graylog2.plugin.configuration.fields.TextField;
 import org.graylog2.plugin.inputs.annotations.Codec;
@@ -119,9 +120,9 @@ public void overrideDefaultValues(@Nonnull ConfigurationRequest cr) {
         public ConfigurationRequest getRequestedConfiguration() {
             final ConfigurationRequest configuration = super.getRequestedConfiguration();
 
-            configuration.addField(new NumberField(CK_CACHE_SIZE, "Maximum cache size", DEFAULT_CACHE_SIZE, "Maximum number of elements in the NetFlow9 template cache"));
-            configuration.addField(new TextField(CK_CACHE_PATH, "Cache file path", DEFAULT_CACHE_PATH, "Path to the file persisting the the NetFlow9 template cache"));
-            configuration.addField(new NumberField(CK_CACHE_SAVE_INTERVAL, "Cache save interval (seconds)", DEFAULT_CACHE_SAVE_INTERVAL, "Interval in seconds for persisting the cache contents"));
+            configuration.addField(new NumberField(CK_CACHE_SIZE, "Maximum cache size", DEFAULT_CACHE_SIZE, "Maximum number of elements in the NetFlow9 template cache", ConfigurationField.Optional.OPTIONAL));
+            configuration.addField(new TextField(CK_CACHE_PATH, "Cache file path", DEFAULT_CACHE_PATH, "Path to the file persisting the the NetFlow9 template cache", ConfigurationField.Optional.OPTIONAL));
+            configuration.addField(new NumberField(CK_CACHE_SAVE_INTERVAL, "Cache save interval (seconds)", DEFAULT_CACHE_SAVE_INTERVAL, "Interval in seconds for persisting the cache contents", ConfigurationField.Optional.OPTIONAL));
 
             return configuration;
         }