nfs: allow protocol family name be configurable

The protocol family name used by LoginBroker and published
as supported transfer protocol by SRM.

Target: trunk, 2.2, 2.6
Ticket: #7233, #7874
Require-notes: yes
Require-book: yes

Acked-by: Paul Millar
Acked-By: Gerd Behrmann
Patch: http://rb.dcache.org/r/5020
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>