Restrict the private network cidr input

The private network creatation did not restrict network cidr. The aim of this blueprint is to let cloud administrator can restrict user private network address cidr input.

Motivation
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Reproduce the error:
1.In admin dashboard networks panel Create a network, check "External Network"
2.Create Subnet, set Network Address "55.115.44.0/24"

3.In project dashboard networks panel Create a network, check "Create Subnet"
 set Network Address "55.115.44.0/24", same as 2. This step can be done successfully.
4.Create Router
5.Two ways to get the exception:
A:
(1).Set the gateway connected to the 1. created public_network. This can be done.
(2).Add interface connected to the 3. created public_network. This can not be done:
Error: Failed to add_interface: Bad router request: Cidr 55.115.44.0/24 of subnet 2958a69d-9a05-483f-a8a0-25f23157cc06 overlaps with cidr 55.115.44.0/24 of subnet a0f4cabb-d708-4907-9083-e92d29ffba0d
B:
(1).Add interface connected to the 3. created public_network. This can be done.
(2).Set the gateway connected to the 1. created public_network. Error:
Error: Failed to set gateway Bad router request: Cidr 55.115.44.0/24 of subnet a0f4cabb-d708-4907-9083-e92d29ffba0d overlaps with cidr 55.115.44.0/24 of subnet 2958a69d-9a05-483f-a8a0-25f23157cc06

It seems that neutron can not overlap ips between private network and public network.
So horizon need to restrict the private network cidr. There is a principle called: never trust user input.