[FLINK-2321] [ml] Set seed for SVM to be random

This closes #889.

docs/libs/ml/svm.md

@@ -181,7 +181,7 @@ The SVM implementation can be controlled by the following parameters:
         <p>
           Defines the seed to initialize the random number generator.
           The seed directly controls which data points are chosen for the SDCA method.
-          (Default value: <strong>0</strong>)
+          (Default value: <strong>Random Long Integer</strong>)
         </p>
       </td>
     </tr>

flink-staging/flink-ml/src/main/scala/org/apache/flink/ml/classification/SVM.scala

@@ -260,7 +260,7 @@ object SVM{
   }
 
   case object Seed extends Parameter[Long] {
-    val defaultValue = Some(0L)
+    val defaultValue = Some(Random.nextLong())
   }
 
   case object ThresholdValue extends Parameter[Double] {