LU-5499 tests: keep /sbin/mount.lustre until cleanup

LU-5499 tests: keep /sbin/mount.lustre until cleanup

Don't unmount /sbin/mount.lustre in the middle of running tests
on a local test system if it is not doing final cleanup.  Otherwise,
later mounts may fail.

The current /sbin/mount.lustre mountpoint is an empty stub that
returns success (0) if executed, but doesn't mount the filesystem.
Instead, create a mountpoint that prints an message if executed and
returns an error to the caller, so it is easier to debug problems.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ied7b69f536bad87333cf5c543384723412500c1e
Reviewed-on: http://review.whamcloud.com/11259
Reviewed-by: frank zago <fzago@cray.com>
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>