NFS live migration in COI has a few caveats.
Live migration requires that the nova uid/gid match across all compute nodes. This is because of the processes across all compute nodes interacting with the disk images in /var/lib/nova/instances.
There is an upstream patch to add the nova user with a user-specified uid/gid prior to the installation of nova-common (the package that creates the nova user). However, there is a bug and/or race condition that somehow nova-common is creating the user prior to the user resource executing. This occurs even when an explicit before is added to the user resource, and require added to the package resource.
So to actually use NFS and live migration, the nova user must be created manually with the specific uid/gid. One this is complete, the puppet run can occur.
The configuration params that need to be set to use NFS in data/hiera_data/user.common.yaml They are internally documented in this file.