Handle scaling / replacing of nodes in conjunction with provisioning ansible / weave network addresses / config

When adding nodes to the cluster we will run into problems with the way we assign the weave bridge address, and scaling. We need to think about the approach for this.
Some options off the top of my head (but needs way more thought)
- form a tigher integration with terraform to derive the values of the bridges / CIDR (possibly).
- use a fact from the machine being provisioned which is dynamic / unique per host
- use the backend service discovery (consul) to populate the values, where consul would hold the master state of the cluster at any point

We also need to handle if nodes are removed / replaced in the cluster, without screwing up the rest of the cluster configuration and breaking the weave network.

This is discussed over here -
https://github.com/Capgemini/Apollo/pull/351#commitcomment-11938353
https://github.com/Capgemini/Apollo/pull/351
https://github.com/Capgemini/Apollo/pull/342