LU-482 test-framework: Ensure dirty cache is flushed before barrier

With certain backend devices like LVM with older kernels the data
in dirty cache cannot be propagated all the way to the block device
with a single sync as there are multiple non-cooperating layers.

So convert such sync calls into triple syncs

Change-Id: If82e25223a277ec165d150b0f5f960ff845af9b0
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1656
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>