[feat] add temporary SSH key generation

The documentation for the `proxmox-iso` plugin stated that when using
the SSH communicator a temporary keypair will be generate if no other
authorization methods are provided.  This operation did not take place
and this commit aims to fix that using the SSH Communicator's
'StepSSHKeyGen' step.

builder/proxmox/common/builder.go

@@ -67,14 +67,22 @@ func (b *Builder) Run(ctx context.Context, ui packersdk.Ui, hook packersdk.Hook,
 		},
 		&commonsteps.StepProvision{},
 		&commonsteps.StepCleanupTempKeys{
-			Comm: &b.config.Comm,
+			Comm: comm,
 		},
 		&stepRemoveCloudInitDrive{},
 		&stepConvertToTemplate{},
 		&stepFinalizeTemplateConfig{},
 		&stepSuccess{},
 	}
 	preSteps := b.preSteps
+	if (*comm).Type == "ssh" {
+		preSteps = append(preSteps,
+			&communicator.StepSSHKeyGen{
+				CommConf:				comm,
+				SSHTemporaryKeyPair:	(*comm).SSHTemporaryKeyPair,
+			},
+		)
+	}
 	for idx := range b.config.ISOs {
 		if b.config.ISOs[idx].ISODownloadPVE {
 			preSteps = append(preSteps,