LU-482 test: sync MDS before first replay test

LU-482 test: sync MDS before first replay test

Maloo autotest system often fails on mountdata lost on MDS, the cause
may be related with LMV and hyporvisor usage. Sync MDS before first
replay to flush all files on MDS.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: I2c0fe37d77b05c81e567c70ef5bffca9eae84632
Reviewed-on: http://review.whamcloud.com/2545
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>