LU-3507 tests: fix sanity test_60a to check run-llog.sh on mgs

Test sanity 60a is skipped if started not from lustre/tests directory.
Test checks run-llog.sh on client, while run-llog.sh is ran on mgs.

This patch checks that run-llog.sh exists on mgs.

Signed-off-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Xyratex-bug-id: MRP-732
Change-Id: I8692e318bb623a8ed9fbaa2a6695c87dc8efeb9e
Reviewed-on: http://review.whamcloud.com/6780
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>