updates to prep demand scaffolding fn

[rpmcginty]

What's in this Change?

This contains updates to the prepare-demand-scaffolding lambda function.

• adding option to specify file system configurations for shared and scratch directories
• adding unit tests
• changing logic in create batch job def and prep args
  • allowing certain public images as they are (previously considered private if not valid ecr uri)

Testing

[njmei]

Just like we an S3URI ValidatedStr class (probably should be renamed to S3Uri), it might be useful to have an EcrUri ValidatedStr class?

[rpmcginty]

We have an ECR URI but it is only for private ecr repos. It might be a little hard because we extract account and region from the registry uri. I was thinking about backporting this, but this might be a little bit difficult.

I can create a separate validated str if you want me to.

[rpmcginty]

okay I will pick up this work in a follow up