Cannot rebuild a networkless container

Description
===========
Create a container and detach its network. The rebuild will fail on this container.

Reproduce
=========
$ CID=$(zun run nginx | awk '/ uuid /{print $4}')
$ zun network-list $CID # print a list of networks of the container
$ zun network-detach $CID private
$ zun network-list $CID # print an empty list
$ zun rebuild $CID
$ zun list # You should see the container failed

zun-compute log: http://paste.openstack.org/show/719335/

Expected
========
The container should rebuild successfully

Actual
======
The rebuild failed