Reverse order of get_unused_ip_addresses

If a test author forgets to disable DHCP on a subnet
used for a test (e.g. I17e201ef8822cace86bf805d6bd5a2e4d0e9084d),
returning the first IP address in the subnet can race with the
creation of the DHCP port. As a defensive measure this adjusts
the logic to get unused IP addresses starting with the last
addresses in the subnet instead.

Change-Id: I31b9fcc9140de08e907bec08584a8c29ec6004e9
1 file changed