LU-8739 test: disable DNE in racer for old MDSs

LU-8739 test: disable DNE in racer for old MDSs

In racer.sh add environtmental variables RACER_ENABLE_REMOTE_DIRS,
RACER_ENABLE_STRIPED_DIRS, and RACER_ENABLE_MIGRATION to enable or
disable the corresponding operations. RACER_ENABLE_REMOTE_DIRS
defaults to true only if the MDS version is at least 2.5, while
RACER_ENABLE_STRIPED_DIRS and RACER_ENABLE_MIGRATION default to true
only if the MDS version is at least 2.8.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I88b6350bd5abef4e13055a0a2ab68692367a6a5d
Reviewed-on: https://review.whamcloud.com/23273
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>