[REEF-1697] Implement JobSubmissionEventImpl.toString()

Add `toString()` method to log all fields of `JobSubmissionEvent` data structure
for better logging.

JIRA:
  [REEF-1697](https://issues.apache.org/jira/browse/REEF-1697)

Pull request:
  This closes #1213

lang/java/reef-common/src/main/java/org/apache/reef/runtime/common/client/api/JobSubmissionEventImpl.java

@@ -57,6 +57,15 @@ private JobSubmissionEventImpl(final Builder builder) {
     this.maxApplicationSubmissions = Optional.ofNullable(builder.maxApplicationSubmissions);
   }
 
+  @Override
+  public String toString() {
+    return String.format(
+        "Job Submission :: user:%s id:%s remote:%s mem:%s queue:%s priority:%s" +
+        " preserve evaluators:%s max submissions:%s global:%s local:%s",
+        this.userName, this.identifier, this.remoteId, this.driverMemory, this.queue, this.priority,
+        this.preserveEvaluators, this.maxApplicationSubmissions, this.globalFileSet, this.localFileSet);
+  }
+
   @Override
   public String getIdentifier() {
     return identifier;