Ignore "unknown instance" error when rm instance

Fixes #4112

Signed-off-by: Jacob Parry <jacob@jacobparry.ca>

drivers/amazonec2/amazonec2.go

@@ -950,6 +950,11 @@ func (d *Driver) terminate() error {
 	_, err := d.getClient().TerminateInstances(&ec2.TerminateInstancesInput{
 		InstanceIds: []*string{&d.InstanceId},
 	})
+
+	if strings.HasPrefix(err.Error(), "unknown instance") {
+		return nil
+	}
+
 	if err != nil {
 		return fmt.Errorf("unable to terminate instance: %s", err)
 	}