Fixed create_port with fixed_ips for public network

Fixed create_port with fixed_ips for public network

RM9593
I wrote a really long comment for the change on lines L413-L428 (red) / L444-467 (green), but then erased it since it wasn't coherent. So, to be brief, I would prefer to see the `zip` processing (specifically `itertools.izip_longest`) happen in `_allocate_ips_from_subnets` instead of creating this complex wrapper. I can talk at length offline about this if you have questions.