SPARK-2058: Overriding SPARK_HOME/conf with SPARK_CONF_DIR

Update of PR #997.

With this PR, setting SPARK_CONF_DIR overrides SPARK_HOME/conf (not only spark-defaults.conf and spark-env).

Author: EugenCepoi <cepoi.eugen@gmail.com>

Closes #2481 from EugenCepoi/SPARK-2058 and squashes the following commits:

0bb32c2 [EugenCepoi] use orElse orNull and fixing trailing percent in compute-classpath.cmd
77f35d7 [EugenCepoi] SPARK-2058: Overriding SPARK_HOME/conf with SPARK_CONF_DIR

(cherry picked from commit f0811f928e5b608e1a2cba3b6828ba0ed03b701d)
Signed-off-by: Andrew Or <andrewor14@gmail.com>