Merge lp://staging/~rvb/maas/bug-1337190 into lp://staging/~maas-committers/maas/trunk

Fix test_updates_mac_cluster_interface: use different interfaces for all the MAC addresses to avoid generating clashing IP addresses.

The problem was that MAC addresses could be linked to the same interface because the interface was chosen at random in:
        mac_addresses = {
            factory.make_mac_address(): random.choice(interfaces)
            for i in range(4)
            }

As a consequence, the call to `get_random_ip_from_interface_range` could return the *same* IP twice in:
        leases = {
            get_random_ip_from_interface_range(interface): (
                mac_address.mac_address)
            for mac_address, interface in mac_addresses.items()
        }
The result was a failure of the whole test because the leases would be missing an entry.

My solution restricts a bit what the test actually tests but I don't think it's a problem in practice because what update_mac_cluster_interfaces() does do not depend on whether the IP addresses in the leases dictionary are on the same interface or not.