Fix for Issue #152

Check if the elastic ip is for VPC and behave appropriately
For `DescribeAddressesResult` it is not mutually exclusive. It will always have the public ip, and will also have the allocation id when it is a VPC elastic IP. 

However for `ec2Service.associateAddress(associateAddressRequest);` it is mutually exclusive; public ip for ec2-classic and allocation id for vpc