Upgrade examples to terraform-google-vm and add test automation

[zefdelgadillo]

• Upgrade module to work with terraform-google-modules/terraform-google-vm
• Upgrade simple example to use managed instance group in terraform-google-modules/terraform-google-vm
• Replace existing test automation with new test automation using kitchen terraform
• Write a test to hit endpoint created
• Review README/documentation (having trouble getting make generate_docs to work correctly)

@morgante at long last, here's the (significant) PR that includes the change to use the new terraform-google-vm managed instance group examples. This included some refactor of the module in addition to adding the example. I also added the test automation that we have on the other newer modules, with a simple test for now that checks for the presence of the forwarding rule. We might want to include a test that checks for the presence of the backend.

I'll squash commits on an approval

closes #12
closes #14

[zefdelgadillo]

Requested changes I think should be resolved, and I opened a few additional issues where there are external dependencies. Added an additional test for presence of load balancer endpoint that gets created. Also updated README documentation to the same standards as some of the other CFT modules.

I'll touch base separately about where I am on adding CI.

[zefdelgadillo]

The integration test I had to test the presence of the load balancer by hitting the endpoint fails for what it seems like the failure of the docker build container to access the internet. The basic functionality of the load balancer in GCP tests for the presence of the backend, but I think to complete an integration test we'd want to test the presence of the front end and forwarding rules.

Are we doing any tests for Internet-facing endpoints like this anywhere else?

control "http" do
  title "http"
    describe http("http://#{attribute("load_balancer_ip")}") do
      its('status') { should eq 200 }
  end
end

Step #3 - "verify":   ×  http: http
Step #3 - "verify":      ×  HTTP GET on http://34.70.209.72 status 
Step #3 - "verify":      execution expired

[Jberlinsky]

@zefdelgadillo We had some full integration tests like what you're describing in other modules, but they've since (to my knowledge, entirely) been removed or commented out because they were "flappy" -- even when they worked, there was no way to guarantee they worked consistently.

[zefdelgadillo]

@morgante or @Jberlinsky thank you both again for the reviews. would like to get a final once over and an approval if all looks good

[morgante]

Some tiny nits, almost there.