Configure agents using neutron.common.config.init (formerly .parse)

After oslo.messaging port is done, we'll need to initialize RPC layer
from all RPC agents. We'll put initialization into init() function, so
the first step for migration is to make agents use it.

The function is renamed to be explicit about the fact that we don't just
parse configuration by calling it, but also do other common
initializations, like setting RPC layer.

blueprint oslo-messaging

Change-Id: I71c62f270ac7a1ff1426a3f49a32133b65580e35
Merge "Configure agents using neutron.common.config.init (formerly .parse)"