Option "Associate Floating IP" available for instances that can't have interfaces with floating ip

[Rafael Ferreira]

If we have one network topology like the one attached, the instance "ne2" has the IP 20.0.0.3 but the 20.0.0.0/24 is not connected to the external network, but when we go to http://host/dashboard/project/instances/<instance-id>/ we see the option "Associate Floating IP" and when the pop-up "Manage Floating IP Associations" opens there aren't any port of instance "ne2" listed, and this is correct. The network that the instance is attached to isn't reachable to an external network, so the option of "Associate Floating IP" is incorrect.

HORIZON_BRANCH=master

[Akihiro Motoki (amotoki)]

Theoretically this can be implemented.

What we need to do are:
- to retrieve a list of routers to gather router interfaces
- to retrieve a list of router interfaces to calculate subnets reachable from routers connected to external network(s)
- to retrieve a list of ports attached to all servers displayed in the instance table
- to calculate which ports belong to the reachable subnets to determine which instances can be associated with FIP

To do this, we need a full list of routers and a full list of ports (of the scoped project).

This might cost and we need to consider a balance between usability and performance.