LU-9336 utils: prevent key clobber and clarify lgss_sk usage

LU-9336 utils: prevent key clobber and clarify lgss_sk usage

Prevent lgss_sk from overwriting key value when modifying attributes.
Altered usage text to match, and clarified that the input source is
the key value, not a source of randomization for key generation.

Signed-off-by: Chris Hanna <hannac@iu.edu>
Change-Id: I87b9d59b65f3172b0425115441eaa1456489daeb
Reviewed-on: https://review.whamcloud.com/26838
Tested-by: Jenkins
Reviewed-by: Kit Westneat <kit.westneat@gmail.com>
Reviewed-by: Nathan Lavender <nblavend@iu.edu>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>