Fixed create_port with fixed_ips for public network

Fixed create_port with fixed_ips for public network

RM9593
Just to add more color on the previous comment, I would change the first few lines of  `_allocate_ips_from_subnets` to look something like this:
```
 def _allocate_ips_from_subnets(self, context, new_addresses, net_id,
                                    subnets, port_id, reuse_after,
                                    ip_addresses=None, **kwargs):
         subnets = subnets or []
         ip_addresses = ip_addresses or []
         for subnet, ip_address in itertools.izip_longest(subnets, ip_addresses):
```
By this point in the execution, len(subnets) will always be >= len(ip_addresses).