Move scheduler_topic into nova.scheduler.rpcapi

The scheduler_topic opt is only used in nova.scheduler.rpcapi and it
makes sense for the RPC module to "own" the topic option.

blueprint: scope-config-opts
Change-Id: Iafb8705eb716b650b63e060fe1dd36666bcee812
Merge "Move scheduler_topic into nova.scheduler.rpcapi"