LU-5986 test: Ensure correct start for conf-sanity/84

For review-zfs:
1) test 79 fails to reformat nodes
2) test 80 fails silently
3) test 84 chokes

Fix test 79 to reformat at end and fix 80 die when it should.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I42c635fffd89eda9ccd6f3f9d739ff8ac75afcf6
Reviewed-on: http://review.whamcloud.com/13016
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>