Currently when using a custom elastic IP, the ENV var `KUBE_MASTER_IP` gets
the output of `$(assign-elastic-ip $ip $master_id)` assigned.
This is wrong since the command returns a string:
`Attaching IP 99.999.999.999 to instance i-9999999`
This patch fixes the assignment by calling `get_instance_public_ip` again.
189f2436e3