Major Description
When I run gremlin through SparkGraphComputer, the SparkUI always displays the Spark App as "Apache TinkerPop's Spark-Gremlin" (see https://github.com/apache/tinkerpop/blob/master/spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/structure/Spark.java#L51) which confused users if I run multiple gremlin with SparkGraphComputer.
It should allow override the Spark App name if user wants to. If user does not specify the Spark app name, we can provide the default one.
