Add support for custom subnetworks

[oralordos]

This should add in support for custom subnetworks while still supporting any of the older flows. A standard custom network still works, and a custom subnetwork will also work.

Fixes #4

[dalekurt]

Thank you @oralordos I truly appreciate the work you have been doing.

[mkell43]

@josh-padnick or other, could this please be merged in? I don't mind using @oralordos's fork, but I would rather be pulling from the official repo.

[brikis98]

@josh-padnick Is on vacation at the moment, but we'll dig into this when he's back next week. Thank you for your patience!

[josh-padnick]

Hi all, apologies for the delay in responding. It wasn't clear how to prioritize reviewing these requests internally and the lack of automated tests made it harder to do so. We've now made the decision to commit to timely responses on the Gruntwork-managed GCP Terraform modules.

For this PR in particular, until GCP support lands in Terratest, we'll need contributors to explicitly verify that they manually tested the changes. In this case, the changes are backwards-compatible, and I validated one set of inputs, so I'll merge now!