Refactor for multiple cloud provider support

[coryodaniel]

• move id/1 to an callback
• Refactor conn/0 token/0 log_http_error/1 into GKE adapter adapter to pave way for pluggable support. conn should be optional to all functions currently accepting it, if not provided, the adapter should call its conn/0 function.
• goth should be an optional dependency and should not automatically be started unless a GCP token is present.
• PoolPolicy spec should be extended to accept spec.provider with the only current valid value gcp
• NodePool should have a provider field.
• Add API for registering an adapter at runtime and looking them up by name
• Ballast.Adapters.Local for development/test
• Move adapter specific HTTP errors into adapter, ie: Tesla.Env in node_pool.ex

defmodule Adapter do
  def register(name, mod), do: "..."
  def lookup(name), do: mod
end

Adapter.register("gcp", Ballast.Adapters.GKE)
Adapter.lookup("gcp") #=> Ballast.Adapters.GKE