builder/googlecompute: template process instance name

builder/googlecompute/config.go

@@ -102,6 +102,7 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		"client_secrets_file": &c.ClientSecretsFile,
 		"image_name":          &c.ImageName,
 		"image_description":   &c.ImageDescription,
+		"instance_name":       &c.InstanceName,
 		"machine_type":        &c.MachineType,
 		"network":             &c.Network,
 		"passphrase":          &c.Passphrase,

website/source/docs/builders/googlecompute.markdown

@@ -96,6 +96,9 @@ Optional:
 
 * `image_description` (string) - The description of the resulting image.
 
+* `instance_name` (string) - A name to give the launched instance. Beware
+  that this must be unique. Defaults to "packer-{{uuid}}".
+
 * `machine_type` (string) - The machine type. Defaults to `n1-standard-1`.
 
 * `network` (string) - The Google Compute network to use for the launched