[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

sbin/spark-config.sh

@@ -20,7 +20,7 @@
 # also should not be passed any arguments, since we need original $*
 
 # resolve links - $0 may be a softlink
-this="${BASH_SOURCE-$0}"
+this="${BASH_SOURCE:-$0}"
 common_bin="$(cd -P -- "$(dirname -- "$this")" && pwd -P)"
 script="$(basename -- "$this")"
 this="$common_bin/$script"