Generate config samples for oslo.messaging

This patch adds oslo.messaging to the
CINDER_CONFIG_GENERATOR_EXTRA_LIBRARIES env variable, which adds
oslo.messaging config options to cinder.conf.sample

Implements blueprint: oslo-messaging

Change-Id: I8aed8a8fdbd5b4ef41e3a3a7dcdebc6f216ec75d
Merge "Generate config samples for oslo.messaging"