Generate interfaces file before cloud-init runs

Cloud-init needs to query the metadata server after the network
interfaces are configured. The upstart job "cloud-init-nonet" is
specifically in place to provide a hook to block cloud-init from
running while we rearrange network interface configurations.

Fixes bug #1233577

Change-Id: Ib5cf75d858fdb670b2abcc082e912c4644d6b169