[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