Comment 1 for bug 1315501

Your 'ask' comment at
  https://ask.openstack.org/en/question/28297/cloud-init-nonet-waiting-and-fails/

/etc/network/interfaces shows:
  # Injected by Nova on instance boot

If that entry was "injected" by nova, then there really isnt much or anything cloud-init can do about this.
This is a good example about why host "injection" is inherently flawed. The right fix for your problem is then to have nova realize that 'eth0' already existed, and remove /etc/interfaces.d/. That is clearly brittle and requires updating your hypervisor/cloud which is quite unreasonable.

if cloud-init reads network-interfaces from config drive, then it should handle eth0 correctl (ie, we need to fix that).