Add TAG_SETTING to list of allowed tags for the ec2 discovery plugin.

I am unable to set ec2 discovery tags because this setting was
accidentally omitted from the register settings list in
Ec2DiscoveryPlugin.java. I get this:

java.lang.IllegalArgumentException: unknown setting [discovery.ec2.tag.project]

plugins/discovery-ec2/src/main/java/org/elasticsearch/plugin/discovery/ec2/Ec2DiscoveryPlugin.java

@@ -134,6 +134,7 @@ public class Ec2DiscoveryPlugin extends Plugin {
         settingsModule.registerSetting(AwsEc2Service.DISCOVERY_EC2.GROUPS_SETTING);
         settingsModule.registerSetting(AwsEc2Service.DISCOVERY_EC2.AVAILABILITY_ZONES_SETTING);
         settingsModule.registerSetting(AwsEc2Service.DISCOVERY_EC2.NODE_CACHE_TIME_SETTING);
+        settingsModule.registerSetting(AwsEc2Service.DISCOVERY_EC2.TAG_SETTING);
     }
 
     /**