Hacky attempt at AWS Provider patching to help users work around Terraform bug

[brikis98]

This PR adds an aws-provider-patch command that can be used to override attributes in nested provider blocks. This is an attempt at a hacky workaround for a Terraform bug where import does not work if you are using any modules that have provider blocks nested in within them. With this PR, users could run:

terragrunt aws-provider-patch --terragrunt-override-attr region=eu-west-1

And Terragrunt will:

1. Run terraform init to download the code for all your modules into .terraform/modules.
2. Scan all the Terraform code in .terraform/modules, find AWS provider blocks, and hard-code the region param
   to eu-west-1 for each one.

Once you do this, you'll hopefully be able to run import on that module. After that, you can delete the modified .terraform/modules and go back to normal.

Note that I wanted to document this command somewhere, so I updated the CLI docs to include all Terragrunt commands, and not just options. Therefore, as a side effect, this PR also fixes #1306.

[brikis98]

I got the patching code working, but there's a bit of an issue: Terraform downloads modules into .terraform/modules, but it downloads the entire repo into that folder, and not just a single module... So the patching code either:

1. Updates every file in the entire repo, which is wasteful, and potentially results in parsing files that perhaps shouldn't be parsed.
2. We have to parse .terraform/modules/modules.json to know exactly in which folders to look... But I'm not sure if that's a stable API we can rely on.

[yorinasub17]

Implementation LGTM, although it's a bit scary to be relying on an internal representation/api.

[brikis98]

Update: I decided to bite the bullet and parse the .terraform/modules/modules.json format after all. Not sure of a better approach.

Note that I wanted to document this command somewhere, so I updated the CLI docs to include all Terragrunt commands, and not just options. Therefore, as a side effect, this PR also fixes #1306. I've updated the description of this PR as well, and I believe it's now ready for review.

[yorinasub17]

LGTM!

[brikis98]

Can I haz one more review plz? 😄

[yorinasub17]

Updates LGTM

[brikis98]

Thanks for the review! Merging now.