Move compute_topic into nova.compute.rpcapi
Move compute_topic into nova.compute.rpcapi

Even though the compute_topic opt is used outside nova.compute.rpcapi,
it makes sense for the RPC module to "own" the topic option.

blueprint: scope-config-opts
Change-Id: I453951825481b9a5ba1865f6bf1223f7b1e25f98

n0ano/ganttclient