lucli update --images

[lucymhdavies]

Borrow from staticli

https://github.com/staticli/staticli/blob/master/cmd/update.go#L18-L32

Need a mechanism to ensure that any images cloned are up-to-date.

I kinda want this to be default behaviour, which would mean an optional override on Cali

https://github.com/skybet/cali/blob/master/docker.go#L193-L197

	if !c.ImageExists(c.Conf.Image){
		if err := c.PullImage(c.Conf.Image); err != nil {
			return "", fmt.Errorf("Failed to fetch image: %s", err)
		}
	}

would be something like

-	if !c.ImageExists(c.Conf.Image){
+	if !c.ImageExists(c.Conf.Image) || task.AlwaysPullImage{

In my case, task.AlwaysPullImage would be set by a flag at the root.