LU-3700 tests: Strengthen and clarify sanity-hsm/test_21

This patch is intended to add full debug and clarify some of
the controls in subtest test_21 of sanity-hsm.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I7546d95c7e90aa9dd213870f3af9d2e9e33c7632
Reviewed-on: http://review.whamcloud.com/8467
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>