Support for multiple DNS zones

[a0s]

Proposed Changes

1. Update the variables.tf to include a new hostname_tag_name variable.
2. Modify main.tf to read hostname_tag_name from the variables instead of hardcoding it.
3. In autoscale.py, change the HOSTNAME_TAG_NAME to read from the os.environ variables.
4. Expand the README.md with a new section explaining how to use the module with multiple DNS zones.
5. Updates to support simultaneous updating of both private and public DNS zones.

Description

I have made updates to the terraform-aws-asg-dns-handler project to add support for multiple DNS zones. This includes changes in the Terraform files and Python script, as well as a comprehensive example and explanation added to the README.md file.

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).