[SPARK-6890] [core] Fix launcher lib work with SPARK_PREPEND_CLASSES.

The fix for SPARK-6406 broke the case where sub-processes are launched
when SPARK_PREPEND_CLASSES is set, because the code now would only add
the launcher's build directory to the sub-process's classpath instead
of the complete assembly.

This patch fixes the problem by having the launch scripts stash the
assembly's location in an environment variable. This is not the prettiest
solution, but it avoids having to plumb that location all the way through
the Worker code that launches executors. The env variable is always
set by the launch scripts, so users cannot override it.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5504 from vanzin/SPARK-6890 and squashes the following commits:

7aec921 [Marcelo Vanzin] Fix tests.
ff87a60 [Marcelo Vanzin] Merge branch 'master' into SPARK-6890
31d3ce8 [Marcelo Vanzin] [SPARK-6890] [core] Fix launcher lib work with SPARK_PREPEND_CLASSES.