Overhaul quantum networking for non-seed clouds.

The networking setup was inappropriate for non-seed clouds - it
depended on two network interfaces, forced NAT that we don't need and
could not be turned off. Additionally there wasn't much clarity
between idempotent machine config and non-idempotent service config.

Key changes in this patch:
* No longer write persistent device files, instead update
  idempotently.
* NAT rules are now setup idempotently on each o-r-c run.
* New definitions for the meaning of various config keys to suit
  working with different cloud layers.

Change-Id: Ie86be0fc884b4c4f655d73da345bdee45fcde473