Graph does not support adding vertices

when I use Hadoop-Gremlin to bulk load json data to JanusGraph,raise such error:
```
org.apache.spark.SparkException: Job aborted due to stage failure: Task 0 in stage 1.0 failed 1 times, most recent failure: Lost task 0.0 in stage 1.0 (TID 1, localhost, executor driver): java.lang.UnsupportedOperationException: Graph does not support adding vertices
at org.apache.tinkerpop.gremlin.structure.Graph$Exceptions.vertexAdditionsNotSupported(Graph.java:1133)
at org.apache.tinkerpop.gremlin.hadoop.structure.HadoopGraph.addVertex(HadoopGraph.java:187)
at org.apache.tinkerpop.gremlin.process.traversal.step.map.AddVertexStartStep.processNextStart(AddVertexStartStep.java:91)
at org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep.next(AbstractStep.java:128)
at org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep.next(AbstractStep.java:38)
at org.apache.tinkerpop.gremlin.process.traversal.util.DefaultTraversal.next(DefaultTraversal.java:200)
at org.apache.tinkerpop.gremlin.process.computer.bulkloading.OneTimeBulkLoader.getOrCreateVertex(OneTimeBulkLoader.java:49)
at org.apache.tinkerpop.gremlin.process.computer.bulkloading.BulkLoaderVertexProgram.executeInternal(BulkLoaderVertexProgram.java:210)
at org.apache.tinkerpop.gremlin.process.computer.bulkloading.BulkLoaderVertexProgram.execute(BulkLoaderVertexProgram.java:197)
at org.apache.tinkerpop.gremlin.spark.process.computer.SparkExecutor.lambda$null$4(SparkExecutor.java:118)
at org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils$3.next(IteratorUtils.java:247)
at scala.collection.convert.Wrappers$JIteratorWrapper.next(Wrappers.scala:43)
at scala.collection.Iterator$$anon$13.hasNext(Iterator.scala:462)
at scala.collection.Iterator$$anon$12.hasNext(Iterator.scala:439)
at org.apache.spark.util.collection.ExternalSorter.insertAll(ExternalSorter.scala:191)
at org.apache.spark.shuffle.sort.SortShuffleWriter.write(SortShuffleWriter.scala:63)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:96)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:53)
at org.apache.spark.scheduler.Task.run(Task.scala:108)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:335)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
```

Reproduce the Hadoop-Gremlin JSON bulk-load failure and trace the reported path through org.apache.tinkerpop.gremlin.hadoop.structure.HadoopGraph.addVertex, OneTimeBulkLoader.getOrCreateVertex, and BulkLoaderVertexProgram.executeInternal. Determine why this path rejects vertex additions in the JanusGraph setup; done means the reported bulk load completes without the UnsupportedOperationException.

Written by the indexing model from the issue text.