Suppress log messages in sbt test with two changes:
1. Set akka log level to ERROR before shutting down the actorSystem.
This avoids akka log messages (like Spray) from falling back to INFO
on the Stdout logger
2. Initialize netty to use SLF4J in LocalSparkContext. This ensures that
stack trace thrown during shutdown is handled by SLF4J instead of stdout
Set DriverSuite log level to WARN
Catch RejectedExecution exception in Checkpoint handler.
Ignore stderr when calling cat on a non-existing file
Remove akka LogLevel fix as we no longer use spray
Merge pull request #683 from shivaram/sbt-test-fix

Remove some stack traces from sbt test output