Improve Packer template

[tpdownes]

• Add support for Shared VPC networks
• Use modern required plugin syntax (similar to Terraform provider version constraints)
• Convert to hashtag comment syntax (following recommendation by tflint)

Submission Checklist

• Have you installed and run this change against pre-commit? pre-commit install
• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?

[cboneti]

One question

[cboneti]

LGTM if the integration tests pass.

[tpdownes]

Integration test is indicating we need an explicit packer init (which was implied before). I will add that to integration test as well as to the instructions printed to screen when using a packer resource.