LU-10629 lod: Clear OST pool with setstripe

LU-10629 lod: Clear OST pool with setstripe

When setstripe -d is run on a directory, we should
clear the OST pool along with all the other settings
Currently there is no way to clear an OST pool,
only change them.

This patch is back-ported from the following one:
Lustre-commit: 37f6357a5c9f4ad0e2269529e7001f5ab63689a5
Lustre-change: https://review.whamcloud.com/31364

Signed-off-by: Ben Evans <bevans@cray.com>
Cray-bug-id: LUS-5696
Change-Id: I50426ce79ab153a715d29cc5d54b0ce70726da41
Reviewed-on: https://review.whamcloud.com/33742
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>