Rewrite GetClusterSubnets() using EC2 specific API

[lanSs]

This PR moves rgt.GetClusterSubnets() to ec2.GetClusterSubnets() and updates the GetClusterSubnets() function to use EC2 specific API instead of Resource Groups Tagging API.

Since there is no VPC endpoint for Resource Groups Tagging API, this change will make the controller be able to run inside an internal VPC subnet without internet gateway or nat gateway.
Please let me know if I need to update anything.

Thanks,

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[lanSs]

/assign M00nF1sh

[M00nF1sh]

/ok-to-test

[lanSs]

/retest

[lanSs]

/retest

[M00nF1sh]

@lanSs
This overall looks good to me and should works.
Just one nit before merge, would you help test change to (I can change this after merge current version if you are not available for this :D)

in := &ec2.DescribeSubnetsInput{Filters: []*ec2.Filter{
		{
			Name:   aws.String("tag:kubernetes.io/cluster/" + c.clusterName),
			Values: aws.StringSlice([]string{"owned", "shared"}),
		},
                {
			Name:   aws.String("tag: + tagSubnetType),
			Values: aws.StringSlice([]string{"", "1"}),
		},
	}}

I believe it should works and more efficient if you have a lot subnets in your VPC. (we had customers with 100+ subnets) :D

[lanSs]

@M00nF1sh
Thank you very much for spending time reviewing it. Your suggestion totally make sense and I have updated the code and the test suite.

Sorry, between my first version and your last review, I should have revisited the hasClusterTagHelper, having the tagSubnetType as parameter make the whole function easier and we don't need the helper function anymore :)

[M00nF1sh]

/approve

[M00nF1sh]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lanSs, M00nF1sh

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