This PR brings support for better log level handling. Terraform logs
are already written in the form which can be understood by the module
https://github.com/hashicorp/logutils .
The TF_LOG environment variable now accepts a log level. Users can pass
levels in the form of "info", "Info" or "INFO" If an invalid log level is
passed, we print a warning but still continue, for backward compatibility
with the old TF_LOG=1 style.
Fatih Arslan