When providing image, allow some default values

[nfachan]

Especially for maelstrom-run, but probably also for test runners, it's nice to be able to shorten the image lines. I can thing of two things we should do.

First, if no use is provided, we should default to everything.

Second, if a string is provided instead of an object, we should expect the value to be thename value for the image, and we should assume a use that of everything.

[nfachan]

We don't want to use everything, since most images don't have a working directory.