Don't instantiate RPC clients on import

Don't instantiate RPC clients on import

In oslo.messaging port, we'll need to make sure no RPC clients or
servers or notifiers are created before RPC layer is initialized using
n_rpc.init(). This means that there should be no global objects that
create those objects on __init__.

There should also be no such class attributes because in that case
import will itself instantiate the object, probably before RPC layer is
ready.

blueprint oslo-messaging

Change-Id: Ia8a9fd39777c75e4253f5518c2de6be551cc365b