Deploy Kops into a Shared VPC

[christopherhein]

For customers hitting a VPC limit we need to document how to use --vpc=${VPC_ID}

https://github.com/kubernetes/kops/blob/master/docs/run_in_existing_vpc.md

[arun-gupta]

@christopherhein send a PR?

[christopherhein]

will do, just documenting for later 👍

[chrislovecnm]

@christopherhein make sure you get the labels part for VPC and subnets. You miss the shared labels and both kops and k8s is NOT happy.