Add topic to the change_node_maintenance_mode() RPC method

Topic is used by the distributed hashing algorithm in order to know to
which conductor it should route the RPC messages to. All the methods in
the rpcapi should support passing a topic to it.

Change-Id: Ida9f2efd07cf5a4f1445f68cdc8aac4e752a8c7a
Merge "Add topic to the change_node_maintenance_mode() RPC method"