Cannot delete network if mtu is changed

[Hongbin Lu]

Create a network, change the mtu to a larger value, then delete the network. The network is not allowed to be deleted with 400 error.

This can be reproduced by the following commands:

$ openstack network create testnet
$ neutron subnet-create testnet --name testsubnet --gateway 10.10.0.1 10.10.0.0/24
$ TOKEN=$(openstack token issue | awk '/ id /{print $4}')
$ NET_ID=$(openstack network show testnet | awk '/ id /{print $4}')
$ curl -g -i -X PUT http://10.0.0.19:9696/v2.0/networks/$NET_ID -H "Content-Type: application/json" -H "X-Auth-Token: $TOKEN" -d '{"network": {"mtu": 9000}}'
$ openstack subnet delete testsubnet
$ openstack network delete testnet
Failed to delete network with name or ID 'testnet': HttpException: Bad Request (HTTP 400) (Request-ID: req-96b39999-e7c2-42c7-9a7c-410fe181f11e), Invalid input for operation: Requested MTU is too big, maximum is 1450.

The failure on deletion is strange. IMHO, we should either validate the mtu on network create/update, or allow the deletion regardless of the mtu value.