testDataChunkFailure[1] failed intermittently on travis.

[zzmao]

The failure:

com.github.ambry.router.GetBlobOperationTest > testDataChunkFailure[1] FAILED
    java.util.concurrent.ExecutionException: com.github.ambry.router.RouterException: Prepare for sending failed Error: UnexpectedInternalError
        at com.github.ambry.router.FutureResult.resultOrThrow(FutureResult.java:113)
        at com.github.ambry.router.FutureResult.get(FutureResult.java:99)
        at com.github.ambry.router.GetBlobOperationTest.doPut(GetBlobOperationTest.java:222)
        at com.github.ambry.router.GetBlobOperationTest.testDataChunkError(GetBlobOperationTest.java:862)
        at com.github.ambry.router.GetBlobOperationTest.testDataChunkFailure(GetBlobOperationTest.java:557)
        Caused by:
        com.github.ambry.router.RouterException: Prepare for sending failed Error: UnexpectedInternalError
            Caused by:
            java.lang.IllegalArgumentException: Total Replica count 1 is less than success target 2