builder/digitalocean: Adhere to new interface, make default in Packer

@pearkes: I added "digital-ocean" to the default config since it will
be shipping with Packer. :)

builder/digitalocean/builder.go

@@ -42,9 +42,12 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raw interface{}) error {
-	if err := mapstructure.Decode(raw, &b.config); err != nil {
-		return err
+func (b *Builder) Prepare(raws ...interface{}) error {
+	for _, raw := range raws {
+		err := mapstructure.Decode(raw, &b.config)
+		if err != nil {
+			return err
+		}
 	}
 
 	// Optional configuration with defaults

config.go

@@ -18,6 +18,7 @@ const defaultConfig = `
 
 	"builders": {
 		"amazon-ebs": "packer-builder-amazon-ebs",
+		"digital-ocean": "packer-builder-digital-ocean",
 		"virtualbox": "packer-builder-virtualbox",
 		"vmware": "packer-builder-vmware"
 	},

plugin/builder-digital-ocean/main.go

@@ -2,7 +2,7 @@ package main
 
 import (
 	"github.com/mitchellh/packer/packer/plugin"
-	"github.com/pearkes/packer/builder/digitalocean"
+	"github.com/mitchellh/packer/builder/digitalocean"
 )
 
 func main() {