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
Merge "Don't instantiate RPC clients on import"