Default log4j properties file should print thread-id in ConversionPattern

The default log4j file in spark/conf/log4j.properties.template doesn't display the thread-id when printing out the logs. It would be very useful to add this, especially for YARN. Currently, logs from all the different threads in a single executor are sent to the same log file. This makes debugging difficult as it is hard to filter out what logs come from what thread.