ec2-simulator: EC2 availability zone

Add availability zone to metadata. It's needed by Cassandra's EC2
multi-region snitch.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cloud-init: Add address resolution to HTTP client

Not all HTTP services can be accessed by an IP address. Fix that up by
using Boost.Asio address resolver.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>