Implement --terragrunt-source-map

[yorinasub17]

Fixes #1138

This implements the --terragrunt-source-map feature described in the ticket linked above. Note the following differences with the issue description:

• --terragrunt-source-map uses standard unix CLI kv definition. That is, it uses --terragrunt-source-map source=dest instead of --terragrunt-source-map source:dest.
• Does not implement file based configuration. The file based configuration can be implemented as an improvement in a future PR if there is demand.

[brikis98]

Very nice!

Just a few questions in the PR. Also, two more here:

1. Does it make sense to deprecate --terragrunt-source?
2. Should we support an env var like TERRAGRUNT_SOURCE_MAP?

[yorinasub17]

Does it make sense to deprecate --terragrunt-source?

Let's wait a little bit for --terragrunt-source-map to get some "in the wild" usage. I am worried there are a few corner cases that are covered by --terragrunt-source that --terragrunt-source-map doesn't cover.

Should we support an env var like TERRAGRUNT_SOURCE_MAP?

What would be the syntax for specifying multiple source mappings via env var?

[brikis98]

Does it make sense to deprecate --terragrunt-source?

Let's wait a little bit for --terragrunt-source-map to get some "in the wild" usage. I am worried there are a few corner cases that are covered by --terragrunt-source that --terragrunt-source-map doesn't cover.

That's a good point.

Should we support an env var like TERRAGRUNT_SOURCE_MAP?

What would be the syntax for specifying multiple source mappings via env var?

Delimiter separated list? The delimiter could be space or semicolon or something else not commonly found in URLs.

[brikis98]

Impl LGTM! Feel free to make the call on env var support (or add in another PR or file issue to track).

[yorinasub17]

Thanks for review! I'll go ahead and merge this in + release it, and then open a new PR for the env var fix.