LU-10505 scripts: avoid version dependent shell script syntax

LU-10505 scripts: avoid version dependent shell script syntax

Stop using -1 as an array index in lfs_migrate.
This isn't a valid construct in all supported bash versions.

Test-Parameters: trivial clientdistro=el6.9

Change-Id: I5500af40926e0fdb2a432c6bae7fbbe05097ec7c
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-on: https://review.whamcloud.com/30860
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>