Optionally pass additional networks to the instance template

[pwillis-eiq]

This passes through the additional_networks variable from https://github.com/terraform-google-modules/terraform-google-vm/tree/master/modules/instance_template . This allows using this module to build a bastion with more than one network interface.

Also passing in optional service_account_email and metadata as I'm using those too, to pass a pre-existing service account, and to pass additional instance metadata.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[comment-bot-dev]

@pwillis-eiq
Thanks for the PR! 🚀
✅ Lint checks have passed.

[bharathkkb]

Thanks for the PR @pwillis-eiq