datasource: replace metadata map with struct

The loosely-typed metadata map is a load of crap. Make it a struct and
let the compiler help us out.

datasource/waagent/waagent.go

@@ -15,13 +15,14 @@
 package waagent
 
 import (
-	"encoding/json"
 	"encoding/xml"
 	"fmt"
 	"io/ioutil"
 	"net"
 	"os"
 	"path"
+
+	"github.com/coreos/coreos-cloudinit/datasource"
 )
 
 type waagent struct {
@@ -46,13 +47,13 @@ func (a *waagent) ConfigRoot() string {
 	return a.root
 }
 
-func (a *waagent) FetchMetadata() ([]byte, error) {
-	metadataBytes, err := a.tryReadFile(path.Join(a.root, "SharedConfig.xml"))
-	if err != nil {
-		return nil, err
+func (a *waagent) FetchMetadata() (metadata datasource.Metadata, err error) {
+	var metadataBytes []byte
+	if metadataBytes, err = a.tryReadFile(path.Join(a.root, "SharedConfig.xml")); err != nil {
+		return
 	}
 	if len(metadataBytes) == 0 {
-		return metadataBytes, nil
+		return
 	}
 
 	type Instance struct {
@@ -74,30 +75,28 @@ func (a *waagent) FetchMetadata() ([]byte, error) {
 		}
 	}
 
-	var metadata SharedConfig
-	if err := xml.Unmarshal(metadataBytes, &metadata); err != nil {
-		return nil, err
+	var m SharedConfig
+	if err = xml.Unmarshal(metadataBytes, &m); err != nil {
+		return
 	}
 
 	var instance Instance
-	for _, i := range metadata.Instances.Instances {
-		if i.Id == metadata.Incarnation.Instance {
+	for _, i := range m.Instances.Instances {
+		if i.Id == m.Incarnation.Instance {
 			instance = i
 			break
 		}
 	}
 
-	attrs := map[string]string{
-		"local-ipv4": instance.Address,
-	}
+	metadata.PrivateIPv4 = net.ParseIP(instance.Address)
 	for _, e := range instance.InputEndpoints.Endpoints {
 		host, _, err := net.SplitHostPort(e.LoadBalancedPublicAddress)
 		if err == nil {
-			attrs["public-ipv4"] = host
+			metadata.PublicIPv4 = net.ParseIP(host)
 			break
 		}
 	}
-	return json.Marshal(attrs)
+	return
 }
 
 func (a *waagent) FetchUserdata() ([]byte, error) {