Set MaxPods when using Amazon VPC CNI Plugin

[ripta]

This PR closes #5510 and replaces it by fixing the merge conflict and implementing the automatic generation for MaxPods, as I've found the original author's patch to be very valuable and would love to see it merged.

• I imported the awsutils package from the VPC CNI plugin with the thought of avoiding maintaining values separate from the plugin. It adds a dependency, so let me know if it's not the best way.
• I added the seen map to machine_types.go, because the Pricing API was doing weird things, where it returned the same instance family multiple times (in some cases, up to 3 times).

Also, @justinsb added a comment in the original PR about possibly lowering the MaxPods. I didn't make any changes here, but let me know if I should try to incorporate that. One one hand, if there are performance penalties with large numbers of pods, it sounds like the defaults should protect cluster operators from shooting themselves in the foot; OTOH, there may(?) exist legitimate use cases that people may have many small pods and want to pack more pods. Should it be a flag to allow folks to opt into running more pods than recommended (110 per node?)

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[chrisz100]

/ok-to-test

Thanks @ripta !

[ripta]

/assign @andrewsykim

Ready to review.

[amitsaha]

Thanks for this @ripta - i was updating the cluster template as part of my workflow (via scripting). This is great default behavior.

[chrisz100]

/lgtm

[andrewsykim]

Not familiar enough in this area to comment :)

/unassign
/assign @justinsb

[justinsb]

Looks great, but I do think we should avoid going >= 110, unless the user explicitly specifies kubelet.MaxPods >= 110.

And in general I think we should do min(kubelet.MaxPods, instanceType.MaxPods)

I also think it would be nice to avoid vendoring all of aws-vpc-cni, so that we can get closer to having this instance type DB move out of kops into a file in some shared repo. Then hopefully aws-vpc-cni and kops and the other projects can all refer to that file :-)

[ripta]

@justinsb - I addressed the comments and resolved the merge conflict. PTAL.

/retest

[justinsb]

Thanks - let's get this into 1.11 then :-)

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrisz100, justinsb, ripta

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