Add support for configuring environment variables on kube-apiserver

[rsafonseca]

This PR allows configuring environment variables for the kube-apiserver.
This presents high value in order to be able to control a number of runtime settings, such as the ones in the code examples.

main motivation:
The apiserver is a memory hungry beast, and currently we have no way of controlling garbage collection settings, which can easily lead to OOM, or the need to run on much larger instances to prevent that, making the kops control plane very costly.

future proofing:
This provides the flexibility to add any other unforeseen variables that could be useful for the kops cluster admin

[k8s-ci-robot]

Hi @rsafonseca. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[hakman]

Cool change 😄
/ok-to-test
/lgtm
/cc @rifelpet @justinsb

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rifelpet

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[hakman]

/test pull-kops-e2e-k8s-gce-cilium