Introduce deisctl and new cluster-aware development environment

This PR introduces `deisctl` to the project and provides a new cluster-aware development environment based on building and pushing Docker images to a private registry.

 * Removes `fleetctl` as a dependency for Deis usage and operation
 * Deprecates the tangled web of `fleetctl` calls in the `Makefile`
 * Refactors `Makefile` targets to use `deisctl`, adds new `dev-registry` and `push` targets
 * Changes to root project README
    * Switch all references of `fleetctl` to `deisctl`
    * Change instructions to provision a 3-node Vagrant by default
    * Clean and shorten content
 * Updates local development instructions
Probably better if we have it check for existence of `docker` binary.