LU-6992 test: wait device to be registered

LU-6992 test: wait device to be registered

in mount_facet, the device label can only be used after the device
registered to MGS and it was rewritten.

Change-Id: I9ed65631391f2be84e484e409fbfe59020d982be
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/16781
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>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>