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.

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

Change-Id: I37a38502362399540c28e78d1343e768b490ce8b
Signed-off-by: Jian Yu <yujian@whamcloud.com>