Add an auth submodule outputting a kubeconfig

[rileykarson]

As well as convenience values for the kubernetes and helm providers.

I'm running the tests following the guide at https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/master/CONTRIBUTING.md#noninteractive-execution, make docker_test_integration has been running since morning though.

/cc @morgante

Fixes #393

[morgante]

@rileykarson Since you're only editing/writing one test, you might want to try using interactive mode to only run that suite. Creating many different GKE clusters is unfortunately indeed very time-consuming.

[bharathkkb]

@rileykarson +1 this is a great addition

I believe you have not added the fixture that invokes the newly added example and hence the int tests are not being called.

For adding a new test you will also need to make an entry here which will let kitchen know the existence of the test. After that you should be able to do kitchen_do list and find the simple_regional_with_kubeconfig test suite.

Then add an entry here which will let cloud build build that particular suite in parallel.

Regarding the test itself maybe invoking kubeclient using the newly generated kubeconfig maybe a good test. We do something similar here but I will defer to @morgante if there is a need to test at all.

[rileykarson]

I added the additional boilerplate, and it appeared to succeed when running in interactive mode. Let me know if you'd like me to exercise the generated kubeconfig.

[bharathkkb]

LGTM