LU-14544 tests: duplicated export entries

LU-14544 tests: duplicated export entries

File /etc/exports could have $MNTPNT entry if
previous NFS tests interrupted.
With duplicated entries in /etc/exports nfs server
service fails to start/restart in RHEL 8 and SLES15.
Patch cleanups exports file before adding $MNTPNT entry.

Test-Parameters: trivial     clientdistro=el8.3 serverdistro=el8.3     testlist=parallel-scale-nfsv3,parallel-scale-nfsv4
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-6291
Reviewed-by: Alexander Lezhoev <alexander.lezhoev@hpe.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Change-Id: I738bd0e8e79dc1ba84e6aa70e06fa47c49a935e0
Reviewed-on: https://review.whamcloud.com/42130
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>