[livy] Support pyFiles properties

With a string or garbabe it boots then stay in running:

curl -X POST --data '{"kind": "pyspark", "pyFiles": "hdfs://localhost:8020/user/romain/dwdwdwpyspark_csv.py"}' -H "Content-Type: application/json" localhost:8998/sessions

        {
            "id": 3,
            "kind": "pyspark",
            "log": [
                "15/10/08 17:56:56 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:56:57 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:56:58 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:56:59 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:00 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:01 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:02 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:03 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:04 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)",
                "15/10/08 17:57:05 INFO yarn.Client: Application report for application_1444345227311_0020 (state: ACCEPTED)"
            ],
           
curl localhost:8998/sessions/2/statements -X POST -H 'Content-Type: application/json' -d '{"code":"1 + 1"}'
java.lang.IllegalStateException: Session is in state starting



I give a list as expected I get:

curl -X POST --data '{"kind": "pyspark", "pyFiles": ["hdfs://localhost:8020/user/romaindwdwd/pyspark_csv.py"]}' -H "Content-Type: application/json" localhost:8998/sessions


        {
            "id": 1,
            "kind": "pyspark",
            "log": [
                "Error: --py-files given but primary resource is not a Python script",
                "Run with --help for usage help or --verbose for debug output"
            ],
            "state": "error"
        },

`spark-submit ... --py-files ...` is explicitly checking that the program it's executing ends with .py. Livy with pyspark actually is executing a jar file, hence this issue. We'll need to find another way to pass along the py-files.