Add empty_groups param to ansible provisioner

provisioner/ansible/provisioner.go

@@ -43,6 +43,7 @@ type Config struct {
 	// The main playbook file to execute.
 	PlaybookFile         string   `mapstructure:"playbook_file"`
 	Groups               []string `mapstructure:"groups"`
+	EmptyGroups          []string `mapstructure:"empty_groups"`
 	HostAlias            string   `mapstructure:"host_alias"`
 	LocalPort            string   `mapstructure:"local_port"`
 	SSHHostKeyFile       string   `mapstructure:"ssh_host_key_file"`
@@ -213,6 +214,10 @@ func (p *Provisioner) Provision(ui packer.Ui, comm packer.Communicator) error {
 			fmt.Fprintf(w, "[%s]\n%s", group, host)
 		}
 
+		for _, group := range p.config.EmptyGroups {
+			fmt.Fprintf(w, "[%s]\n", group)
+		}
+
 		if err := w.Flush(); err != nil {
 			tf.Close()
 			return fmt.Errorf("Error preparing inventory file: %s", err)

website/source/docs/provisioners/ansible.html.markdown

@@ -51,6 +51,9 @@ Optional Parameters:
   should be placed. When unspecified, the host is not associated with any
   groups.
 
+- `empty_groups` (array of strings) - The groups which should be present in
+  inventory file but remain empty.
+
 - `host_alias` (string) - The alias by which the Ansible host should be known.
   Defaults to `default`.