NetworkNotFound should be raised when a network is not found in network manager

[Nachi Ueno]

db.network_get_by_cidr() can return None.
In that case some method in network manager such as delete_network raises AttriuteError.
But this is not informative. In this case, Network manager should throw exception.

Following methods have same problem.
- FloatingIP.associate_floating_ip()
- NetworkManager._get_dhcp_ip()
- NetworkManager.allocate_fixed_ip()
- NetworkManager.deallocate_fixed_ip()
- NetworkManager.lease_fixed_ip()
- NetworkManager.release_fixed_ip()
- NetworkManager.validate_networks()
- VlanManager.allocate_fixed_ip()

[Nachi Ueno (nati-ueno)]

https://github.com/ntt-pf-lab/nova/tree/openstack-qa-nova-872522

[Mark McLoughlin (markmc)]

Moving from InProgress to Confirmed since nobody is assigned

[Ivan Kolodyazhny (e0ne)]

Looks like this issue is already fixed. Need someone more to verify it

[Tom Fifield (fifieldt)]

Believe this 3 year old bug is long-fixed. If the bug recurs, please reopen.