Use scala found in PATH by default if it's the right version

This commit allows for a smoother out-of-the-box experience by not requiring the user to create a `conf/spark-env.sh` file if they already have an apropriate `scala` in their `PATH`. 

This is done by setting `SPARK_LAUNCH_WITH_SCALA` if `SCALA_HOME` is unset and a suitable `scala` is found.