LU-797 tests: improve test_23b of ost-pools.sh

LU-797 tests: improve test_23b of ost-pools.sh

- fail the test once the written space is greater than the
  available space.
- use it's own maxfree limit, and increase the limit a lot.
- terminate the test when the write speed is too slow.

Test-Parameters: envdefinitions=SLOW=yes testlist=ost-pools
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Id9aef77ef07358a03133ed6d34c299609540ef42
Reviewed-on: http://review.whamcloud.com/4831
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>