Allow Kubeconfig to be more configurable

[sdavids13]

PR o'clock

Description

Making Kubeconfig significantly more configurable in the following ways:

• Setting the cluster name to the same value as the EKS cluster (so cluster name doesn't conflict with the old kubernetes naming)
• Allowing the user name to be modified
• Allowing the context name to be modified
• Allowing additional heptio parameters to be specified (useful to be able to specify a role that should be assumed)
• Allowed environment variables to be specified for the heptio execution, this provided a generic implementation compated to adding capability to use non-default aws profile #44.

Checklist

• terraform fmt and terraform validate both work from the root and examples/eks_test_fixture directories (look in CI for an example)
• Tests for the changes have been added and passing (for bug fixes/features)
• Test results are pasted in this PR (in lieu of CI)
• Docs have been added/updated (for bug fixes/features)
• Any breaking changes are noted in the description above

[brandonjbjelland]

This seems like the most flexible way I've seen thus far to add configuration while still providing a minimal interface to get started. LGTM

[max-rocket-internet]

I think this should default to the cluster name as aws is very uninformative.

[sdavids13]

Yea, I do agree (I configured my context name to be the cluster name as well). I was trying to limit the amount of changes made by default but if others think that is a more reasonable default I would be happy to provide another PR to fix it.

[sdavids13]

Also, I just found that if you are trying to merge various EKS kubeconfigs together via the KUBECONFIG environment variable the username needs to be unique across all of the config files. So, I will go ahead and make the change for both the context name and the user name.

[brandonjbjelland]

That'd all be appreciated @sdavids13 . Thanks for driving that

[max-rocket-internet]

username needs to be unique across all of the config files

Correct.

I've made a PR: #52

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