Support for input variables through OS environment variables and command-line

[RafaelPalomar]

Is your feature request related to a problem? Please describe.

Luet currently has the possibility to specify variables through value files (--values <file> flag). This scheme could be enhanced by adding support for setting variables independently through either a application flags (e.g., --var <variable_definition>) or environment variables (e.g., export LUET_mypackage_testing=OFF).

Describe the solution you'd like
I would like to see the same variables handling implemented in Terraform:

• through files, this is similar to what Luet currently supports.
• through OS environment variables
• through command line

Describe alternatives you've considered

Additional context