When MASTER_RESERVED_IP is set to elastic IP from AWS, then aws/util.sh will
associate it with master instance and assign it to KUBE_MASTER_IP. If no MASTER_RESERVED_IP
is set, new elastic ip will be requested from amazon. This allows cluster certificates to
be generated for an IP that doesn't change between stopping & starting cluster instances.
The requested elastic ip is not released when kube-down.sh is run. I think it is good
because user could have created DNS records and it would be bad if the IP was removed.
He can reuse it next time through MASTER_RESERVED_IP when setting up cluster again.
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