LU-9170 test: fix for hostlist_expand

LU-9170 test: fix for hostlist_expand

Fix string substitution in hostlist_expand. It
may produce a wrong result in some case.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: Iba33833ec4955dafeeefe5042508a0162c2d477e
Reviewed-on: https://review.whamcloud.com/25698
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>