LU-11495 tests: zconf_mount_clients() fix to work with FILESET

If FILESET is set zconf_mount_clients() fails to mount clients
because of missing $mnt dir on the clients.

When FILESET is set the following tests are to be skipped:
- tests accessing .lustre directly
- tests involving lustre_rsync (which needs .lustre to do
  open by FID)
- tests involving copytool (lhsmtool_posix needs .lustre)
- tests taking use of llapi_open_by_fid (sanity.sh:test_405()
  ->swap_lock_test, sanity.sh:test_807()->llsom_sync)
- tests require Lustre root mounted on $MOUNT
  (sanity.sh:test_65n() needs root to test default layout
  inheritance)

Test-Parameters: trivial envdefinitions=FILESET=/subdir
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-6546, LUS-6561
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Change-Id: I68081fbff808abbaddb314e36283e09d151a81db
Reviewed-on: https://review.whamcloud.com/33338
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>