Traffic Ops should prohibit deleting the last Server in a Cache Group that is assigned to a Topology

[zrhoffman]

I'm submitting a ...

• new feature / enhancement request

Traffic Control components affected ...

• Traffic Ops

Current behavior:

Servers can be deleted or removed from a Cache Group used by a Topology, even if it means that Cache Group would no longer have any Servers in it.

#5144 already added this validation for POST and PUT /api/3.0/topologies, so if such a Server was deleted or moved to a different Cache Group, updating that Topology would then fail, as long as the Topology still contains that Cache Group.

New behavior:

As a CDN operator, I would like to prevent deleting the last server in a Cache Group that is assigned to any Topology. With no Servers in a Cache Group assigned to a Topology, that Topology is no longer valid, so the Cachegroup should be removed from the Topology before the last server can be deleted from it.

Minimal reproduction of the problem with instructions:

For DELETE:

1. Make a Cache Group
2. Add only 1 Server to the Cache Group
3. Make a Topology with that Cache Group
4. Delete the Server (should fail)

For PUT:

1. Make a Cache Group
2. Add only 1 Server to the Cache Group
3. Make a Topology with that Cache Group
4. Move the Server to a different Cache Group (should fail)