AWS: root volume provisioned IOPS support

[yoz2326]

It is possible to set rootVolumeType: gp2 so an instance group will use gp2 as disk type (https://github.com/kubernetes/kops/blob/master/docs/instance_groups.md)

If the option is set to rootVolumeType: io1 then the below error is thrown when building the cluster:

W0731 13:44:44.298875 8784 executor.go:109] error running task "LaunchConfiguration/master-eu-west-1a.masters.cluster.name" (9m48s remaining to succeed): error creating AutoscalingLaunchConfiguration: ValidationError: Iops is required for a provisioned IOPS volume. status code: 400, request id: 072e23b2-75ee-11e7-968a-191d6f6e9343

This PR tries to address this so if anyone needs, it can use io1 as disk type for the root volume.
In order to define the number of Iops set rootVolumeIops: <number> (minimum is 100)

If rootVolumeType: io1 is set and rootVolumeIops is not defined, then rootVolumeIops defaults to 100.

I am not quite familiar with the Kops codebase and this is my first encounter with Go, so some changes might not be suitable (I also do not have a software developer background so bear with me :) ). Please review and add feed-back.

I've included a small change to the Makefile as I compiled Kops on a Mac and didn't had $GOPATH set after installing Go. I thought that pulling this via go env would be more flexible, but I am not 100% sure as I don't do much development in this area and not sure about possible use cases.

[k8s-ci-robot]

Hi @yoz2326. Thanks for your PR.

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[yoz2326]

/assign @chrislovecnm

[justinsb]

/ok-to-test

[justinsb]

/lgtm

[justinsb]

This is great - thanks @yoz2326

[yoz2326]

/approve no-issue

[justinsb]

/approve no-issue

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb, yoz2326

Associated issue requirement bypassed by: justinsb

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[yoz2326]

/test pull-kops-e2e-kubernetes-aws

[yoz2326]

/test pull-kops-e2e-kubernetes-aws

[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue