webadmin: wait untill billing cell is ready

StandardBillingService should wait untill billing cell is ready

Ticket: #7981
Acked-by: Paul Millar
Acked-by: Albert Rossi
Target: 2.6, 2.7, master
Require-book: no
Require-notes: no
Merge pull request #182 from kofemann/2.6

webadmin: wait untill billing cell is ready