make sample config generation work with oslo messaging

[Joe Gordon]

From: https://review.openstack.org/#/c/39929

"All of the messaging options are now removed from nova.conf.sample because the sample config file generator doesn't yet know how to pick up these."

We need this fixed before we release Icehouse

[Davanum Srinivas (DIMS) (dims-v)]

ceilometer uses the EXTRA_MODULES_FILE/oslo.config.generator.rc support in tools/config/generate_sample.sh to pick up config options from keystoneclient.middleware.auth_token, we can do the same for oslo.messaging when that patch gets in.

[Doug Hellmann (doug-hellmann)]

I believe the work done for https://blueprints.launchpad.net/oslo/+spec/improve-config-discovery-for-docs should be useful for resolving this, and commit 6fd09137f8c45ce4790833d7f6b840d885818771 brought the feature of the config generator in to nova.

[Rohan (kanaderohan)]

I think this patch https://review.openstack.org/#/c/39929 has taken care of loss of config options by using the config generator options in nova.

This is not a open issue now.