Add `--stack-generate` to `find` and `list`

[yhakbar]

Describe the enhancement

Automatically generate all stacks in the current working directory when using the --stack-generate flag of find and list.

Additional context

Do not consider implementation of this feature prior to the release of Terragrunt 1.0.

Should be pretty easy to implement.

RFC Not Needed

• I have evaluated the complexity of this enhancement, and I believe it does not require an RFC.