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/builder/hosts/$ip/host
    /deis/builder/hosts/$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.
feat(deisctl): support scaling the builder