[simulator] Fix a message length generator usage
[simulator] Fix a message length generator usage

Do initialization of the generator only for rpc/notify clients.
No needs to load messages_length.yaml and calculate distribution for servers.

Change-Id: Ib1ac30181bec20eacf6d29ccd5f76c0b6b1e96f8

ozamiatin/oslo.messaging,ozamiatin/oslo.messaging