SPARK-4924 addendum. Minor assembly directory fix in load-spark-env-sh

Set the current dir path $FWDIR and same at $ASSEMBLY_DIR1, $ASSEMBLY_DIR2
otherwise $SPARK_HOME cannot be visible from spark-env.sh -- no SPARK_HOME variable is assigned there.
I am using the Spark-1.3.0 source code package and I come across with this when trying to start the master: sbin/start-master.sh

Author: raschild <raschild@users.noreply.github.com>

Closes #5261 from raschild/patch-1 and squashes the following commits:

b9babcd [raschild] Update load-spark-env.sh