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.

Signed-off-by: Ben Evans <bevans@cray.com>
Cray-bug-id: LUS-5696
Change-Id: I50426ce79ab153a715d29cc5d54b0ce70726da41
Reviewed-on: https://review.whamcloud.com/31364
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>