EC2 seed provider

Cassandra cluster auto-configuration on EC2 uses the Datastax reflector
service for discovering seed nodes. Instead of relying on external
scripts, this patch implements EC2 seed provider that uses the Datastax
reflector service.

This is particularly useful for EC2 AMIs that don't include a complete
userspace (such as those built with OSv) that can update Cassandra
configuration during boot.

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