cloud-init: External and internal IP accessors

This adds external and internal IP accessors to the data_source class.
These will be exposed to the Cassandra configuration module templating
engine.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
scripts/ec2-simulator: Public IP address

Add public IP address to EC2 metadata.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cloud-init: External and internal IPs templating

Add external and internal IP addresses as template markers
{{external_ip}} and {{internal_ip}}, respectively. These are needed to
configure IP addresses for Cassandra EC2 multi-region snitch.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>