Updates for x1 instance family

[manabusakai]

This PR adds support for the "x1.16xlarge" instance type.
ref: https://aws.amazon.com/ec2/pricing/on-demand/

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[chrislovecnm]

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[manabusakai]

@chrislovecnm
I have just signed the CLA. Let me know if I have missed any prereq!

[justinsb]

/ok-to-test

/lgtm

Thanks @manabusakai

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

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