Auto tuning feature enhancements (linkedin#379)

Auto tuning feature enhancements:

Tuning auto switch off when:
        * parameter converges
        * the maximum number of tuning iterations is reached
        * no gain in cost function
Remembers and returns the best parameter set when:
        * tuning is switched off
        * no new parameter suggestion exists for the job
        * execution has failed and a retry is attempted
Parameters being tune now attain only discrete values defined by the step size
Bug fixes

app-conf/SchedulerConf.xml

@@ -30,7 +30,7 @@
         <classname>com.linkedin.drelephant.schedulers.AzkabanScheduler</classname>
         <params>
           <!--<exception_enabled>true</exception_enabled>-->
-          <!--<exception_class>com.linkedin.drelephant.exceptions.azkaban.AzkabanWorkflowClient</exception_class>-->
+          <!--<workflow_client>com.linkedin.drelephant.exceptions.azkaban.AzkabanWorkflowClient</workflow_client>-->
           <!--<private_key>/home/key/private_key</private_key>-->
           <!--<userame>elephant</userame>-->
           <!--<password></password>-->