Neutron: Fix subnet allocation_pools

Asking for the last address of the pool can raise HTTPError 500 Server Error (Internal Server Error).

To reproduce it:

• create a network
• create a subnet
• create a port with fixed_ips = the second to last address of the pool
• create a port with fixed_ips = the last address of the pool

Here are the subnets before the last command:
{
"subnets" : [ {
"id" : "84cc68d5-f91f-40bb-9766-eca1b5f28bad",
"network_id" : "15ae6e88-d9bc-45aa-82d9-aa1ef7cdbdef",
"name" : "",
"ip_version" : 4,
"cidr" : "10.0.0.0/24",
"dns_nameservers" : [ ],
"allocation_pools" : [

{ "start" : "10.0.0.1", "end" : "10.0.0.8" }

,

{ "end" : "10.0.0.10" }

],
"host_routes" : [ ],
"enable_dhcp" : false,
"tenant_id" : "2cd9944ab92a4840a59407a8db7f3934",
"ipv6_address_mode" : null,
"ipv6_ra_mode" : null
} ]
}