LU-17152 tests: unmount NFS clients with zconf_umount_clients

LU-17152 tests: unmount NFS clients with zconf_umount_clients

This patch fixes cleanup_nfs() to unmount NFS clients by running
zconf_umount_clients(), which can find and kill active processes
that are accessing the NFS mount point so as to avoid the
"device is busy" failure.

The patch also adds racer_on_nfs test into always_except list for
parallel-scale-nfsv4 due to LU-17154.

Lustre-change: https://review.whamcloud.com/52533
Lustre-commit: TBD (from 52a2147e8b0eca74f38b1b87991b53ccf25663cd)

Test-Parameters: trivial testlist=parallel-scale-nfsv4

Change-Id: I37a38502362399540c28e78d1343e768b490ce8b
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52534
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>