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.go

@@ -0,0 +1,22 @@
+// Package autocert is the ACME interpreter from golang.org/x/crypto/acme/autocert
+package autocert
+
+import (
+	"net"
+
+	"github.com/micro/go-micro/api/server/acme"
+	"golang.org/x/crypto/acme/autocert"
+)
+
+// autoCertACME is the ACME provider from golang.org/x/crypto/acme/autocert
+type autocertProvider struct{}
+
+// NewListener implements acme.Provider
+func (a *autocertProvider) NewListener(ACMEHosts ...string) (net.Listener, error) {
+	return autocert.NewListener(ACMEHosts...), nil
+}
+
+// New returns an autocert acme.Provider
+func New() acme.Provider {
+	return &autocertProvider{}
+}