Start abstracting away the ACME provider (#830)

* Start abstracting away the ACME provider

* Move ACME to interface with sub-package implementations

* Addressing comments

* Library -> Provider

* Missed a couple of Library -> Provider

* One more Library -> Provider

* remove constants

api/server/acme/acme.go

@@ -0,0 +1,16 @@
+// Package acme abstracts away various ACME libraries
+package acme
+
+import (
+	"errors"
+	"net"
+)
+
+var (
+	ErrProviderNotImplemented = errors.New("Provider not implemented")
+)
+
+// Provider is a ACME provider interface
+type Provider interface {
+	NewListener(...string) (net.Listener, error)
+}