The Ceilometer middleware for Swift uses oslo.messaging under the hood. Specifically, it takes the url config option and calls oslo_messaging.get_transport, which accepts the following format:

transport://user:pass@host1:port[,hostN:portN]/virtual_host

This is described in detail in the http://docs.openstack.org/developer/oslo.messaging/transport.html (oslo.messaging documentation). For your example, you would configure the Ceilometer filter like so:

[filter:ceilometer]
paste.filter_factory = ceilometermiddleware.swift:filter_factory
control_exchange = swift
url = rabbit://user:password@controller1:5672,controller2:5672,controller:5672/vhost
driver = messagingv2
topic = notifications
log_level = WARN