LU-8394 tests: exportfs unexport failure can be ignored

LU-8394 tests: exportfs unexport failure can be ignored

During NFS test cleanup, 'service nfs stop' is called before
'exportfs -u', while the former one should have unexport all
already, so the failure of the latter can be ignored.

Test-Parameters: trivial alwaysuploadlogs \
envdefinitions=ONLY=compilebench \
clientdistro=el7 ossdistro=el7 mdsdistro=el7 \
testlist=parallel-scale-nfsv3,parallel-scale-nfsv4

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I4a6931202dccde2909cab6c60c3ebc002fe4fd98
Reviewed-on: http://review.whamcloud.com/21290
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>