devstack/plugin.sh: stop using deprecated option group for rabbit
devstack/plugin.sh: stop using deprecated option group for rabbit

oslo_messaging has created separate config file sections for
messaging configuration.
refer to:
https://review.openstack.org/#/c/219356/

Change-Id: I1ceec1657d2d0b1b1df5ebfb8c85fd3d977afdfa

StackStorm/mistral,openstack/mistral,openstack/mistral,StackStorm/mistral