LU-5031 tests: obdfilter-survey fixes

There are couple of problems in obdfilter-survey:
- Type of test brw i.e. "g" was not followed with npages,
- Target netdisk was not set properly and
- Turn ON async flag only for mode 3.
This patch fixed the above mention problems.

Signed-off-by: Rahul Deshmukh <rahul_deshmukh@xyratex.com>
Change-Id: I4360b52d1b8d828def6cf316abc09383fcaaaa94
Reviewed-on: http://review.whamcloud.com/10264


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Cliff White <cliff.white@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>