Generate Heat Template from existing Network Topology

= Motivations

Today a user can snapshots instances to have backups or reusable images. He can also snapshots volumes.
And in the future he will even be able to snapshot a complete Heat stack.

But what about networks? He can't snapshot an existing topology.

= Description

The idea is to be able to generate a complete Heat network template from an existing network topology. This Heat feature would be the way of snapshotting networks when a topology is designed without Heat.

The user can deploy the template again, in other tenants or even on another OpenStack platform with few changes, to find back the topology.

This is also a great way for a beginner to write a first heat template by actually configuring via Horizon networks, subnets, routers, etc, as if he were using Horizon as a Heat visual editor.