[HOTFIX] Wait for EOF only for the PySpark shell

In `SparkSubmitDriverBootstrapper`, we wait for the parent process to send us an `EOF` before finishing the application. This is applicable for the PySpark shell because we terminate the application the same way. However if we run a python application, for instance, the JVM actually never exits unless it receives a manual EOF from the user. This is causing a few tests to timeout.

We only need to do this for the PySpark shell because Spark submit runs as a python subprocess only in this case. Thus, the normal Spark shell doesn't need to go through this case even though it is also a REPL.

Thanks davies for reporting this.

Author: Andrew Or <andrewor14@gmail.com>

Closes #2170 from andrewor14/bootstrap-hotfix and squashes the following commits:

42963f5 [Andrew Or] Do not wait for EOF unless this is the pyspark shell
(cherry picked from commit dafe343499bbc688e266106e4bb897f9e619834e)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>