LU-15060 tests: sanity-flr test_208[a,b] fix

LU-15060 tests: sanity-flr test_208[a,b] fix

sanity-flr  test_208a  and test_208b failed as:
  test_208a returned 2
  test_208b returned 2
on Lustre setup where osts are located not on one host
because of stack_trap "do_nodes $osts $LCTL set_param $old"
returns 2. Let's use save_lustre_params() instead of trying to
set not-existing parameters.

Fixes: 8507472dd37e ("LU-14996 lov: prefer mirrors on non-rotational OSTs")
Test-Parameters: trivial testlist=sanity-flr env=ONLY=208
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Change-Id: I19cedc0a9745d0d112ac05fe3a800347ab4c40d3
Reviewed-on: https://review.whamcloud.com/45130
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>