Added ipv6 option for digitalocean builder

the ipv6 option is already part of the godo package

Updated documentation to reflect new feature

Closes: https://github.com/hashicorp/packer/issues/5533

builder/digitalocean/config.go

@@ -28,6 +28,7 @@ type Config struct {
 
 	PrivateNetworking bool          `mapstructure:"private_networking"`
 	Monitoring        bool          `mapstructure:"monitoring"`
+	IPv6              bool          `mapstructure:"ipv6"`
 	SnapshotName      string        `mapstructure:"snapshot_name"`
 	SnapshotRegions   []string      `mapstructure:"snapshot_regions"`
 	StateTimeout      time.Duration `mapstructure:"state_timeout"`

builder/digitalocean/step_create_droplet.go

@@ -47,6 +47,7 @@ func (s *stepCreateDroplet) Run(state multistep.StateBag) multistep.StepAction {
 		},
 		PrivateNetworking: c.PrivateNetworking,
 		Monitoring:        c.Monitoring,
+		IPv6:              c.IPv6,
 		UserData:          userData,
 	})
 	if err != nil {

website/source/docs/builders/digitalocean.html.md

@@ -69,6 +69,9 @@ builder.
 -   `monitoring` (boolean) - Set to `true` to enable monitoring
     for the droplet being created. This defaults to `false`, or not enabled.
 
+-   `ipv6` (boolean) - Set to `true` to enable ipv6
+    for the droplet being created. This defaults to `false`, or not enabled.
+
 -   `snapshot_name` (string) - The name of the resulting snapshot that will
     appear in your account. This must be unique. To help make this unique, use a
     function like `timestamp` (see [configuration