nfs: add pnfs/nopnfs export option

there are situations, when we would like to disable
pnfs functionality and proxy the data through a door.

requesting 0.9 to get it into dcache-2.11 branch

Acked-by: Paul Millar
Target: master, 0.9
(cherry picked from commit bcb5e248bf420f646e38c8adea122d00e18cd813)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>