LU-1526 tests: Support for MDS-initiated OST_DESTROYs

LU-1526 tests: Support for MDS-initiated OST_DESTROYs

This patch is backported from commit af5f388 of LU-1303 to
support interoperating with 2.4 server.

The patch makes sure the tests work with MDSs that destroy OST
objects asynchronously on behalf of clients.

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I8d8cb9e3699b6e7f63af106a5f45363f61f3ce7c
Reviewed-on: http://review.whamcloud.com/4959
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>