Updates the vpc-info object with a lookup for IP -> subnet.

def _make_ip_subnet_lookup(vpc_info):
    """
    Updates the vpc-info object with a lookup for IP -> subnet.

    """
    # We create a reverse lookup from the instances private IP addresses to the
    # subnets they are associated with. This is used later on in order to
    # determine whether routes should be set in an RT: Is the RT's subnet
    # associated with ANY of the IP addresses in the route spec? To make this
    # easy, we collect the IPs and subnets of all EC2 instances in the VPC.
    # Once we get a route spec, we create a list of subnets for only the
    # cluster nodes. The assumption is that not all EC2 instances in the VPC
    # necessarily belong to the cluster. We really want to narrow it down to
    # the cluster nodes only. See make_cluster_node_subnet_list().
    vpc_info['ip_subnet_lookup'] = {}
    for instance in vpc_info['instances']:
        for interface in instance.interfaces:
            subnet_id = interface.subnet_id
            for priv_addr in interface.private_ip_addresses:
                vpc_info['ip_subnet_lookup'][priv_addr.private_ip_address] = \
                                                                    subnet_id