spark-submit with accept multiple properties-files and merge the values

Current ```spark-submit``` accepts only one properties-file, and use ```spark-defaults.conf``` if unspecified.
A more nature approach is patching the properties-files sequentially against ```spark-defaults.conf```.

This PR affairs:
1. spark-submit script: join multiple ```--properties-file``` with comma and stored as ```SPARK_SUBMIT_PROPERTIES_FILES``` environment variable. Peek each properties-file to set ```SPARK_SUBMIT_BOOTSTRAP_DRIVER``` flag.
2. SparkSubmitArguments.scala: similar with 1.
3. SparkSubmitDriverBootstrapper.scala: accept ```SPARK_SUBMIT_PROPERTIES_FILES``` and call ```getPropertiesFromFiles``` for parsing.
4. Utils.scala: add ```getPropertiesFromFiles``` for the parsing of multiple properties-files.