LU-1625 test: reduce test duration for nfs mode

LU-1625 test: reduce test duration for nfs mode

There isn't much value to run long duration in
nfs mode.  Based on original work by Minh Diep.

Test-Parameters: testgroup=full
Signed-off-by: Keith Mannthey <keith@whamcloud.com>
Change-Id: I635d388e4dba5192199602b29ccaae843e9a1346
Reviewed-on: http://review.whamcloud.com/3596
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>