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>
[SPARK-3696]Do not override the user-difined conf_dir

https://issues.apache.org/jira/browse/SPARK-3696

We see if SPARK_CONF_DIR is already defined before assignment.

Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #2541 from WangTaoTheTonic/confdir and squashes the following commits:

c3f31e0 [WangTaoTheTonic] Do not override the user-difined conf_dir

(cherry picked from commit 9d320e222c221e5bb827cddf01a83e64a16d74ff)
Signed-off-by: Andrew Or <andrewor14@gmail.com>

Conflicts:
	sbin/spark-config.sh
[SPARK-3535][Mesos] Fix resource handling.

Author: Brenden Matthews <brenden@diddyinc.com>

Closes #2401 from brndnmtthws/master and squashes the following commits:

4abaa5d [Brenden Matthews] [SPARK-3535][Mesos] Fix resource handling.

(cherry picked from commit a8c52d5343e19731909e73db5de151a324d31cd5)
Signed-off-by: Andrew Or <andrewor14@gmail.com>
[SPARK-3775] Not suitable error message in spark-shell.cmd

Modified some sentence of error message in bin\*.cmd.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2640 from tsudukim/feature/SPARK-3775 and squashes the following commits:

3458afb [Masayoshi TSUZUKI] [SPARK-3775] Not suitable error message in spark-shell.cmd

(cherry picked from commit 358d7ffd01b4a3fbae313890522cf662c71af6e5)
Signed-off-by: Andrew Or <andrewor14@gmail.com>
[SPARK-3774] typo comment in bin/utils.sh

Modified the comment of bin/utils.sh.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2639 from tsudukim/feature/SPARK-3774 and squashes the following commits:

707b779 [Masayoshi TSUZUKI] [SPARK-3774] typo comment in bin/utils.sh

(cherry picked from commit e5566e05b1ac99aa6caf1701e47ebcdb68a002c6)
Signed-off-by: Andrew Or <andrewor14@gmail.com>