LU-14996 lov: prefer mirrors on non-rotational OSTs

LU-14996 lov: prefer mirrors on non-rotational OSTs

consider non-rotational OSTs as preferred unless explicit prefer
flag is set on a mirror.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I787bcba0b5e45842c9d4762c7f97a8f44a4fc9cb
Reviewed-on: https://review.whamcloud.com/44883
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>