0.19.0: Adds check to only write backend config file if path doesn't exist

what

• This allows users to edit the backend config file once written and not have to worry about it being overwritten by the module due to changes.

why

• Any changes to the backend.tf file are overwritten on the next apply, which can be annoying if trying to specify additional required_providers, required_version, or other terraform configuration block changes. This updates the template file block so that file is not overwritten if the backend.tf (or other terraform config file name) already exists.