KAFKA-3774: Make 'time' an optional argument of GetOffsetShell

Since the 'time' argument has a default value of -1, it makes sense to make it an optional argument.

Author: Vahid Hashemian <vahidhashemian@us.ibm.com>

Reviewers: Ewen Cheslack-Postava <ewen@confluent.io>

Closes #1457 from vahidhashemian/KAFKA-3774

core/src/main/scala/kafka/tools/GetOffsetShell.scala

@@ -64,7 +64,7 @@ object GetOffsetShell {
 
     val options = parser.parse(args : _*)
 
-    CommandLineUtils.checkRequiredArgs(parser, options, brokerListOpt, topicOpt, timeOpt)
+    CommandLineUtils.checkRequiredArgs(parser, options, brokerListOpt, topicOpt)
 
     val clientId = "GetOffsetShell"
     val brokerList = options.valueOf(brokerListOpt)