MRP-1622 tests: improve parallel-scal-nfs cleanup

parallel-scale-nfs does not cleanup the nfs setup in case
of failure which breaks next tests.

Add trap to perform cleanup in case of any error.
Add possibility to specify nfs server's lustre mount point
and mount points for nfs clients.

Signed-off-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Change-Id: I42e8388e9922a0ee21100891cada941d1a4cdc54
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/3578
Reviewed-by: Vladimir Saveliev <vladimir_saveliev@xyratex.com>
Reviewed-by: Andriy Skulysh <andriy_skulysh@xyratex.com>
Tested-by: Jenkins
Tested-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>