Update /cachegroups endpoints for Topologies

[zrhoffman]

I'm submitting a ...

• new feature / enhancement request

Description

The /cachegroups endpoints need to be updated to reflect the changes that having Topologies introduces.

Traffic Control components affected ...

• Documentation
• Traffic Ops

Acceptance Criteria

• GET /cachegroups supports ?topology={{topology}} as a query parameter to retrieve all the Cachegroups that are used in a given Topology.
• DELETE /cachegroups returns a useful error when trying to delete a Cachegroup that is currently used in a Topology.
• PUT /cachegroups updates all foreign key references if name is updated. This is done automatically in the database via the FK references.
  The line

See the Flexible Topologies PR for specifics: #4537