LU-4093 tests: prevent zombie requests when stopping CT

LU-4093 tests: prevent zombie requests when stopping CT

Fix to prevent any current requests to become zombie when stopping
copytool and thus not to sometimes to induce failures during next
sanity-hsm sub-tests.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I3274e5864469251e6f2040377b84429e4f218f47
Reviewed-on: http://review.whamcloud.com/8157
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>