LU-5110 test: use different dryrun option in sanity-scrub.sh

Use --dryrun on/off in sanity-scrub.sh test_15 scripts to
work with 2.5.x interop tests.

Test-Parameters: envdefinitions=SLOW=yes,ONLY=15 testlist=sanity-scrub
Test-Parameters: envdefinitions=SLOW=yes,ONLY=15 testlist=sanity-scrub \
ossjob=lustre-b2_5 mdsjob=lustre-b2_5 ossbuildno=61 mdsbuildno=61
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I11fba4510b16fbf52aa2591574d5d1c71805a132
Reviewed-on: http://review.whamcloud.com/10621
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>