Added billing property

Specifies the cell address services submit billing info to. Not all
services respect this property (at least statistics and poolmanager
don't).

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5605/