Add ability to use ec2 nat instance as egress

[relu]

Hello folks, I bumped into the of lack of support for NAT instances as egress and here's what I came up with to help support it. I'm not sure if this is the right approach, even though the solution worked in my test case.

Mind you, this is my first time working with golang so please bear with me. I'd be more than happy to help out move this forward with a bit of guidance.

This relates to #1530.

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[relu]

/assign @justinsb

[justinsb]

This is awesome :-)

So I was wondering if we should build an instance object vs just changing the Route.Instance field to an string (it seems to currently be unused), but I like what you've done.

One gotcha is that currently the Instance will be tagged I believe; we probably don't want to do that when we specify it by ID (I think it's more likely to get deleted for example). I'll send a PR to change Instance to take a list of Tags, which is the way we've settled on doing this now :-)

/approve

/lgtm

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

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[justinsb]

/ok-to-test