webadmin: wait untill billing cell is ready

StandardBillingService should wait untill billing cell is ready

Ticket: #7981
Acked-by: Albert Rossi
Acked-By: Paul Millar
Target: 2.6, 2.7, master

Require-book: no
Require-notes: no
Merge pull request #183 from kofemann/2.7

webadmin: wait untill billing cell is ready