LU-15334 tests: cleanup conf-sanity test_30a

LU-15334 tests: cleanup conf-sanity test_30a

Fix typo: use error() instead of not existing fail(),
localize some variables.

Fixes: 5e546603cb ("b=15253 add conf_param -d to remove permanent settings")
Test-Parameters: trivial testlist=conf-sanity env=ONLY=30a
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Change-Id: I970d8422ba8ba75aca922d8ac6bac09c7cfcd67d
Reviewed-on: https://review.whamcloud.com/45811
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>