Don't pass RPC connection to pre_start_hook

In review I5bf7795fca21627566ef4f688d45dc83bb953d1b (commit 3349417) we
passed an rpc_connection argument to pre_start_hook() so that we could
create additional queues.

See review Idf12c418a8ce1bc873e7ad6f702351e95d31aca3 for how this was
intended to be used. With oslo.messaging, we'd support doing something
like this by adding support for multiple targets to RPCServer since the
concept of a messaging connection isn't really exposed by the API in
quite the same way.

In any case, this rpc_connection parameter has never been used since it
was introduced, so let's remove it. Let's also move the invocation of
this hook back to where it originally was.

blueprint: oslo-messaging
Change-Id: I9181a3567a1b5a8a6077e78c85f4d28660f275a6
Merge "Don't pass RPC connection to pre_start_hook"