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/url/url.go

@@ -15,6 +15,7 @@
 package url
 
 import (
+	"github.com/coreos/coreos-cloudinit/datasource"
 	"github.com/coreos/coreos-cloudinit/pkg"
 )
 
@@ -40,8 +41,8 @@ func (f *remoteFile) ConfigRoot() string {
 	return ""
 }
 
-func (f *remoteFile) FetchMetadata() ([]byte, error) {
-	return []byte{}, nil
+func (f *remoteFile) FetchMetadata() (datasource.Metadata, error) {
+	return datasource.Metadata{}, nil
 }
 
 func (f *remoteFile) FetchUserdata() ([]byte, error) {