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/autocert/autocert_test.go

@@ -0,0 +1,15 @@
+package autocert
+
+import (
+	"testing"
+)
+
+func TestAutocert(t *testing.T) {
+	l := New()
+	if _, ok := l.(*autocertProvider); !ok {
+		t.Error("New() didn't return an autocertProvider")
+	}
+	if _, err := l.NewListener(); err != nil {
+		t.Error(err.Error())
+	}
+}