poolmanager: Made billing cell address configurable for pool manager

Addresses the issue that the billing cell address for pool manager
was injected through admin commands (rc set warning path) rather
than through the configuration system.

Obsoletes the admin command 'rc set warning path'.

Target: trunk
Require-notes: yes
Require-book: yes
Patch: http://rb.dcache.org/r/5606/
Acked-by: Paul Millar <paul.millar@desy.de> (in person)
Acked-by: Tigran Mkrtchya <tigran.mkrtchyan@desy.de>