Fix SpringOpenTTP thread leak

- It was creating 2 theads everytime a Device connected.
  Now uses thread pool shared across Devices, where threads will die out on idle.
  Should resolve ONOS-3579

Change-Id: I490b2ef677853677fbd151af27f6ac2be563774c
diff --git a/drivers/default/src/main/java/org/onosproject/driver/pipeline/SpringOpenTTPDell.java b/drivers/default/src/main/java/org/onosproject/driver/pipeline/SpringOpenTTPDell.java
index e3afc0b..2774349 100644
--- a/drivers/default/src/main/java/org/onosproject/driver/pipeline/SpringOpenTTPDell.java
+++ b/drivers/default/src/main/java/org/onosproject/driver/pipeline/SpringOpenTTPDell.java
@@ -15,6 +15,8 @@
  */
 package org.onosproject.driver.pipeline;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
@@ -42,6 +44,7 @@
 import org.onosproject.net.flowobjective.ForwardingObjective;
 import org.onosproject.net.flowobjective.ObjectiveError;
 import org.onosproject.net.group.Group;
+import org.slf4j.Logger;
 
 /**
  * Spring-open driver implementation for Dell hardware switches.
@@ -55,6 +58,8 @@
     private static final int DELL_TABLE_MPLS = 25;
     private static final int DELL_TABLE_ACL = 40;
 
+    private final Logger log = getLogger(getClass());
+
     //TODO: Store this info in the distributed store.
     private MacAddress deviceTMac = null;