[SPARK-4076] Parameter expansion in spark-config is wrong

In sbin/spark-config.sh, parameter expansion is used to extract source root as follows.

    this="${BASH_SOURCE-$0}"

I think, the parameter expansion should be ":" instead of "".
If we use "-" and BASH_SOURCE="", (empty character is set, not unset),
"" (empty character) is set to $this.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2930 from sarutak/SPARK-4076 and squashes the following commits:

32a0370 [Kousuke Saruta] Fixed wrong parameter expansion