Low: extend support for AWS elastic ip association

make it possible to associate secondary private ip address with elastic
ip address, if allocation-id.

if private-ip-address were both set, it will try to associate elastic
interface with the allocation-id, otherwise, it will try to associate
elastic ip with private ip instead (default).

reference docs:
http://docs.aws.amazon.com/cli/latest/reference/ec2/associate-address.html