Make some builder config usage more consistent with other builders

builder/digitalocean/config.go

@@ -42,10 +42,10 @@ type Config struct {
 }
 
 func NewConfig(raws ...interface{}) (*Config, []string, error) {
-	var c Config
+	c := new(Config)
 
 	var md mapstructure.Metadata
-	err := config.Decode(&c, &config.DecodeOpts{
+	err := config.Decode(c, &config.DecodeOpts{
 		Metadata:    &md,
 		Interpolate: true,
 		InterpolateFilter: &interpolate.RenderFilter{
@@ -142,5 +142,5 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	}
 
 	common.ScrubConfig(c, c.APIToken)
-	return &c, nil, nil
+	return c, nil, nil
 }

builder/docker/config.go

@@ -31,10 +31,10 @@ type Config struct {
 }
 
 func NewConfig(raws ...interface{}) (*Config, []string, error) {
-	var c Config
+	c := new(Config)
 
 	var md mapstructure.Metadata
-	err := config.Decode(&c, &config.DecodeOpts{
+	err := config.Decode(c, &config.DecodeOpts{
 		Metadata:    &md,
 		Interpolate: true,
 		InterpolateFilter: &interpolate.RenderFilter{
@@ -91,5 +91,5 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		return nil, nil, errs
 	}
 
-	return &c, nil, nil
+	return c, nil, nil
 }

builder/virtualbox/ovf/config.go

@@ -40,8 +40,8 @@ type Config struct {
 }
 
 func NewConfig(raws ...interface{}) (*Config, []string, error) {
-	var c Config
-	err := config.Decode(&c, &config.DecodeOpts{
+	c := new(Config)
+	err := config.Decode(c, &config.DecodeOpts{
 		Interpolate: true,
 		InterpolateFilter: &interpolate.RenderFilter{
 			Exclude: []string{
@@ -132,5 +132,5 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		c.ImportFlags = append(c.ImportFlags, "--options", c.ImportOpts)
 	}
 
-	return &c, warnings, nil
+	return c, warnings, nil
 }