As https://github.com/mesos/spark/pull/292 points out, the Typesafe Scala RPM installs the scala executable in /usr/bin/ and places the Scala library JARs in /usr/share/java/.
That pull request added a SCALA_LIBRARY_PATH setting to support this method of installing Scala.
We should document proper configuration of SCALA_LIBRARY_PATH and SCALA_HOME; right now, that pull request is the only documentation and it's implicit that the proper Debian / CentOS settings are SCALA_HOME=/usr/ and SCALA_LIBRARY_PATH=/usr/share/java/.
I propose adding extra comments to the spark-conf.sh template that describe typical values for these settings. Maybe we also want to mention this in the docs somewhere?