comply with changes after terraform v0.11 (> v0.12)

[saadismail]

This PR fixes #2, I am testing it locally on:

#terraform --version
Terraform v0.14.6
+ provider registry.terraform.io/hashicorp/google v3.55.0

For now, this PR is a draft, will open it for your review once I am done testing it throughly.

[saadismail]

I had made a silly mistake while populating instances' public IP in provisioners. Now, I am able to get all the infra up with out any issues, have some issues with kubectl connecting to controllers, will get back once I figure this out.

[saadismail]

GTG, I have got everything working perfectly now. I have also added a note about SSH key comment in README.md, being not experienced with GCP, I figured it out after a couple of hours.

@aidanSoles hope you can review it and merge it sometime. Also thank you for this amazing project, it must have helped a lot of people play around with kubernetes.

[aidanSoles]

Hey @saadismail! Thanks so much for doing this PR. Sorry for the delay in reviewing! I'll check it out by the end of the week! And I hope it has been helpful!

[saadismail]

Hey @aidanSoles, no rush but possible to get this merged? I can see that #4 also does the same thing and possibly more people will do the duplicated work again in the future.

[aidanSoles]

Hey there @saadismail! My apologies for the delay! No excuse--my bad on not getting to it.

I have a chunk of time tomorrow where I'm getting back on the bandwagon for personal projects. I'll plan on testing the changes then. At first glance though, the PR looks pretty good :+1:! May I have you do one of the following though (depending on what you think is make the most sense)?

1. rebase your changes so that all the interpolation syntax changes are in one commit, and the lock file change is in another.
2. rebase all your changes into one commit.

Thanks again so much for this PR (and your patience)!