Close #710, fix ALKilnInThePlayground upload Step

[plocket]

A few quirks here.

1. This doesn't allow passing in command line arguments for source folder paths yet. That may have to be a separate feature. I'm not sure it should be a required command line argument anyway, in which case we'll still need that code for times when the argument isn't handed in. We could ensure ALKilnInThePlayground hands it in, but I'm not yet sure how we can get ALKilnInThePlayground to get the right paths. Maybe it would have better access to the S3 paths, though.
2. [Added a fix in the in-Playground interview that makes this point moot. See https://github.com/Close #18, cache files for S3 docassemble-ALKilnInThePlayground#24] It doesn't quite work with S3. Jonathan gave us the info that the /tmp folder for S3 won't keep those files around, so we probably can't count on those files being there for running these tests in an interview (ALKilnInThePlayground - now it occurs to me that ALKilnInterview may have been a better name...)
3. I should have probably broken one change into a different PR and I can do that if this is too much for one PR - The variable setting Step for uploading files turned out to be a mismatch for one ethos we're following. We decided that we want to defer to being radically permissive and forgiving with test code in general. In that spirit, I changed it so that if ALKiln can't find a file that the author listed to upload, the file just isn't uploaded and, if not all variables are required to be set, the test can continue. In a Story Table, the fields are not required to be set and there would be no error just because of a missing file. In a linear Step ("I set the var..."), a missing file would cause an error. I'm not sure about being this permissive, but I've been unsure in similar situations in the past and the collective decision has generally erred on the side of flexibility.

I'll fix the version upon publishing.

Closes #710

[plocket]

Not super simple, but could use a look (if you have time), @BryceStevenWilley.

[BryceStevenWilley]

LGTM, a few small thoughts mentioned in the comments.