LU-9014 test: handle separate MGS in conf-sanity test_72()

LU-9014 test: handle separate MGS in conf-sanity test_72()

In conf-sanity test_72() when using a separate MGS, add --replace to
the mkfs options to prevent failures from already registered targets.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I06a266a8ca954c4b7c62fee3957145854d48dd38
Reviewed-on: https://review.whamcloud.com/24852
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>