LU-6900 tests: parallel-scale-nfs improvement

LU-6900 tests: parallel-scale-nfs improvement

This patch addresses a set of parallel-scale-nfs.sh suite
defects and fixes:
1. Suite deals only with nfs server exported from SINGLEMDS
(lustre_client) and nfs clients set to default CLIENTS list.
-- Patch adds the possiblity to specify NFS_SERVER, NFS_CLIENTS,
   nfs server's lustre mount point and mount points for nfs
   clients; functions which run applications operate with
   nfs client mntpt instead of global $DIR.

2. Suite does not cleanup the nfs setup in case
of failure, this breaks next tests.
-- Patch adds trap to perform cleanup in case of any error.

3. Suite can not run on solo setup because it requires nfs
clients and nfs servers are to be the different nodes.
-- Patch skips the suite for singe node setup.

Test-Parameters: mdtcount=1 ostsizegb=2 testlist=parallel-scale-nfsv3,parallel-scale-nfsv4
Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Seagate-bug-id: MRP-1742, MRP-1622, MRP-2056, MRP-2682
Reviewed-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Reviewed-by: Alexander Lezhoev <alexander.lezhoev@seagate.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Change-Id: I3e1c877764db561f7e8ca7703b90cc5cf6f91ca0
Reviewed-on: https://review.whamcloud.com/15712
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>