LU-3858 test: fix sanity 27A failure caused by stripe delay

LU-3858 test: fix sanity 27A failure caused by stripe delay

When clearing the stripe of root directory, the updating of stripe
might be delayed because of the processing delay of config log.
This will cause the failure of sanity 27A test.

This makes the test suit wait for the update of root directory
stripe.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: Ib4f96d9d4d0ae8a0b54e3e306a7abed758dbdf39
Reviewed-on: http://review.whamcloud.com/7495
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>