feat(builder): support multiple builders

This extends the builder path to allow multiple deis-builders. For an upgrade path please make sure to upgrade both the router and the builder at the same time.

This functions by making the following change to the etcd paths:

    /deis/builder/host
    /deis/builder/port

now becomes:

    /deis/builders/$ip/host
    /deis/builders/$ip/port

This is repeated for each host running the deis builder and `$ip` is replaced by the private IP address for the CoreOS node.
This should follow the same naming conventions we use in the router: https://github.com/deis/deis/blob/master/router/bin/boot#L15-L16