LU-10566 test: fix nfs exports clean up

LU-10566 test: fix nfs exports clean up

nfsv3 clean up fail causing subsequence tests to fail

Test-Parameters: trivial testlist=parallel-scale-nfsv3, parallel-scale-nfsv4

Change-Id: Ic4eb02ea4e4f142753fe36cf110ea0fbec398822
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/31679
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>