LU-15707 lod: force creation of a component without a pool

LU-15707 lod: force creation of a component without a pool

This patch add the pool option "lfs setstripe -p ignore" to force
the creation of component without a pool set by inheritance (from
parent or root).

e.g:
$ lfs setstripe -p pool tdir
$ lfs setstripe -E1M -p ignore -E-1 -p '' -c2 -S2M tdir/tfile
$ lfs getstripe -I1 -p tdir/tfile
(no pool set)
$ lfs getstripe -I2 -p tdir/tfile
pool
(inherited from tdir)

This patch add the test "ost-pools test_32" to verify this behavior.

The poorly-named "-p none" keyword, which indicates the pool name
should be inherited from the root or parent dir layout, will be
eventually replaced by the new "-p inherit" keyword.

Test-Parameters: serverdistro=el7.9 serverversion=2.12.8 testlist=ost-pools env=ONLY=32,ONLY_REPEAT=50
Test-Parameters: testlist=ost-pools env=ONLY=32,ONLY_REPEAT=50
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I782cbafe209cff6857162303a4650f5e3b438be5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/46955
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>