Added unit tests and some minor changes for ProxyArp.

Change-Id: I027919c331575ba57b3d342683d73e1c31284810
diff --git a/src/main/java/net/onrc/onos/apps/proxyarp/ArpCache.java b/src/main/java/net/onrc/onos/apps/proxyarp/ArpCache.java
index 38cd41f..6216f59 100644
--- a/src/main/java/net/onrc/onos/apps/proxyarp/ArpCache.java
+++ b/src/main/java/net/onrc/onos/apps/proxyarp/ArpCache.java
@@ -95,6 +95,10 @@
         log.debug("Set arpEntryTimeoutConfig {}", ArpCache.arpEntryTimeoutConfig);
    }
 
+    public long getArpEntryTimeout() {
+        return ArpCache.arpEntryTimeoutConfig;
+    }
+
     /**
      * Get the MAC address that is mapped to an IP address in the ARP cache.
      *