LU-2284 tests: clear limits before testing

LU-2284 tests: clear limits before testing

The quota limits left over from other tests which has quota enabled
could confuse the sanity-quota, so we'd clear all the before testing.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Ie24e6891d920a0c7fa6ffc29ee8ff9400b8636af
Reviewed-on: http://review.whamcloud.com/4766
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>