Remove final qualifier to facilitate unit testing

Author: russgold

operator/src/main/java/oracle/kubernetes/operator/helpers/ClientPool.java

@@ -17,7 +17,7 @@
 
 public class ClientPool extends Pool<ApiClient> {
   private static final LoggingFacade LOGGER = LoggingFactory.getLogger("Operator", "Operator");
-  private static final ClientPool SINGLETON = new ClientPool();
+  private static ClientPool SINGLETON = new ClientPool();
 
   private static final ClientFactory FACTORY = new DefaultClientFactory();