Updated openstack/openstack
Project: openstack/ceilometer  4857fcd0344ddedbf6a149c12506963ab88d2eef

remove reference to model in migration

the model represents the current active representation.
migrations on the other hand represent some moment in past that
may or may not reflect the current model. we shouldn't reference
model unless we have to.

Change-Id: I78dca9043014acc1a6eda479f687f63d31ce1ea7