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
pom: bump java source/target to 1.8

Acked-by: Paul Millar
Target: master