Support tfenv+tgenv

[AlainODea]

Invoke terraform and terragrunt from the directory of the file being checked. This allows tfenv and tgenv to load the correct versions of terraform and terragrunt in a mixed-version repository.

This is for the same reasons as gruntwork-io/module-ci#150:

tfenv loads the correct terraform version based on a file called .terraform_version in the present working directory. Prior to this change, terraform-update-variable would use the root of the cloned repo as the present working directory and select a potentially incompatible version of terraform.

[brikis98]

Thanks for the PR! I'm super booked this week, but will try to take a look in the next few days.

[yorinasub17]

This makes sense to me! Found one minor issue, but other than that should be able to merge this in.

[yorinasub17]

Thanks! Will merge and release now!