Let backend choose number of IO threads by default.

e.g., Netty NIO uses # cores * 2 threads.

Change-Id: I59b125aaae50fb067214d067a06a98d258726a26
diff --git a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/Controller.java b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/Controller.java
index deb95f3..5fce725 100644
--- a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/Controller.java
+++ b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/Controller.java
@@ -80,7 +80,7 @@
 
     // Configuration options
     protected List<Integer> openFlowPorts = ImmutableList.of(6633, 6653);
-    protected int workerThreads = 16;
+    protected int workerThreads = 0;
 
     // Start time of the controller
     protected long systemStartTime;
diff --git a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OpenFlowControllerImpl.java b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OpenFlowControllerImpl.java
index f44c625..c07328f 100644
--- a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OpenFlowControllerImpl.java
+++ b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OpenFlowControllerImpl.java
@@ -88,7 +88,7 @@
 public class OpenFlowControllerImpl implements OpenFlowController {
     private static final String APP_ID = "org.onosproject.openflow-base";
     private static final String DEFAULT_OFPORT = "6633,6653";
-    private static final int DEFAULT_WORKER_THREADS = 16;
+    private static final int DEFAULT_WORKER_THREADS = 0;
 
     private static final Logger log =
             LoggerFactory.getLogger(OpenFlowControllerImpl.class);
@@ -111,7 +111,7 @@
     private String openflowPorts = DEFAULT_OFPORT;
 
     @Property(name = "workerThreads", intValue = DEFAULT_WORKER_THREADS,
-            label = "Number of controller worker threads; default is 16")
+            label = "Number of controller worker threads")
     private int workerThreads = DEFAULT_WORKER_THREADS;
 
     protected ExecutorService executorMsgs =