"auto assign" on unmanaged subnet used the gateway IP for the node

[Andreas Hasenack]

maas 1.9.0+bzr4533-0ubuntu1~trusty1

On a node with two nics, I chose an unmanaged subnet for the second nic and selected "auto assign". After deployment, that nic got the same IP as the subnet's gateway. See attached screenshot.

Here, with the node still deployed:
andreas@nsn7:~$ maas admin-atlas subnet statistics subnet-13 include_ranges=true
Success.
Machine-readable output follows:
{
    "total_addresses": 254,
    "usage_string": "0%",
    "num_available": 253,
    "ranges": [
        {
            "start": "10.96.130.1",
            "end": "10.96.130.1",
            "purpose": [
                "assigned-ip"
            ],
            "num_addresses": 1
        },
        {
            "start": "10.96.130.2",
            "end": "10.96.130.254",
            "purpose": [
                "unused"
            ],
            "num_addresses": 253
        }
    ],
    "largest_available": 253,
    "num_unavailable": 1,
    "usage": 0.003937007874015748,
    "inefficiency_string": "TBD",
    "available_string": "100%"
}

Same call with the node released:
andreas@nsn7:~$ maas admin-atlas subnet statistics subnet-13 include_ranges=true
Success.
Machine-readable output follows:
{
    "total_addresses": 254,
    "usage_string": "0%",
    "num_available": 254,
    "ranges": [
        {
            "start": "10.96.130.1",
            "end": "10.96.130.254",
            "purpose": [
                "unused"
            ],
            "num_addresses": 254
        }
    ],
    "largest_available": 254,
    "num_unavailable": 0,
    "usage": 0.0,
    "inefficiency_string": "TBD",
    "available_string": "100%"
}

[Mike Pontillo (mpontillo)]

This does not affect MAAS 2.0, as IP allocation has been completely rewritten.

This bug occurs because the Subnet calls out to each NodeGroupInterface to determine if dynamic ranges, static ranges, cluster IP addresses, gateway IP addresses, and DNS server are reserved on the subnet.

In the case of an unmanaged subnet *without* a corresponding NodeGroupInterface, the code improperly skips values that can be determined by looking at the subnet alone (that is, gateway IP and DNS servers). These should be returned directly from the Subnet rather than going to the NodeGroupInterface.