Allow overriding images

[jacobtomlinson]

We could make it possible to override images in the crontab too. Currently if you want to use a different image you need to create a new template. However the rest of the template may be totally fine, you just want a different image.

How would this work?

• It could be another document level yaml comment like the template. Should it affect every command below? Should it just affect the next command? Should it be consistent with the template specification/
• It could be another in line yaml comment. How do you add multiple keys in one line? Use JSON notation instead of the yaml notation? 0 1 * * * echo hello # {"name": "test", "image": "myimage"}
• Should image and tag be specified as two keys or a combined string?