Ensure minimum Terragrunt version

[robkinyon]

(From #172)

To ensure consistent versioning across a team, please provide a way to specify the minimum Terragrunt version within configuration.

[brikis98]

Thanks!

[brikis98]

Probably the best way to handle this is with two new settings in the Terragrunt configuration:

terragrunt = {
  terragrunt_version_constraint = ">= 0.12.0"
  terraform_version_constraint = ">= 0.9.3"
}

[robkinyon]

@brikis98 What about just the following with _constraint?

terragrunt = {
  terragrunt_version = ">= 0.12.0"
  terraform_version = ">= 0.9.3"
}

I think that reads better.

[brikis98]

Agreed, that's cleaner.

[brikis98]

We now have:

1. terragrunt_version_constraint
2. terraform_version_constraint