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.

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[comment-bot-dev]

@pwillis-eiq Thanks for the PR! 🚀
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