Traffic Ops should prohibit adding a cache group with no servers to a topology

[zrhoffman]

I'm submitting a ...

• new feature / enhancement request

Traffic Control components affected ...

• Traffic Ops

Current behavior:

You can create a topology that contains a cachegroup with no servers in it, even though a delivery service that uses such a topology will not work.

New behavior:

Users should be prohibited from creating topologies whose cachegroups do not each contain at least one server.

Minimal reproduction of the problem with instructions:

1. Make topology with servers in it
2. Delete servers, then update the topology. This should return a 400 response but returns a 200 response.