Fixed create_port with fixed_ips for public network

Fixed create_port with fixed_ips for public network

RM9593
I agree with your idea of using itertools.izip_longest, which would simplify a lot of things here. But I think that has to be done in allocate_ip_address method itself. Because, we need to select_subnet or choose_available_subnet based on individual ip_address. When it comes to _allocate_ips_from_subnets method, subnets is not None. 

If I have to zip and loop in _allocate_ips_from_subnets, then I have to also select_subnet inside this method, as I dont know what ip_address to use to select a subnet.