Obtain Public IP address for EC2 instance
Obtain Public IP address for EC2 instance

Because an `ifconfig` does not show the public IP address of an EC2
instance, we have to do it through the instance metadata, as seen here:

    http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html

Signed-off-by: Pang Yan Han <9d8a3bce71455786d192cd4b985538c54e03f109@viki.com>

viki-org/storm-docker,viki-org/storm-docker