store Spark properties like we do JVM properties

[scottdraves]

in beakerx.json:

    "spark_options": {
      "spark.executor.memory": "8g",
      "spark.master": "local[*]",
      "spark.executor.cores": "10",
      "user": "strings"
    }

should be:

    "spark_options": {
      "spark.executor.memory": "8g",
      "spark.master": "local[*]",
      "spark.executor.cores": "10",
      "properties": [
                {
                    "name": "user",
                    "value": "strings"
                }
            ]
    }

like the JVM options work.