Use cachegroupIndex to find cachegroup type, not parentIndex

[zrhoffman]

What does this PR (Pull Request) do?

• This PR fixes Traffic Ops Topology with Secondary Parent returns error #4772

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Make Rob's cachegroups, POST his topology, and make sure it succeeds:

{
  "name": "Foo",
  "description": "bar",
  "nodes": [
    {
      "cachegroup": "mid-west",
      "parent": "",
      "secParent": "",
      "parents": []
    },
    {
      "cachegroup": "sacramento-ca-usa",
      "parent": "mid-west",
      "secParent": "mid-east",
      "parents": [
        0,
        2
      ]
    },
    {
      "cachegroup": "mid-east",
      "parent": "",
      "secParent": "",
      "parents": []
    },
    {
      "cachegroup": "albany-ny-usa",
      "parent": "mid-east",
      "secParent": "mid-west",
      "parents": [
        2,
        0
      ]
    },
    {
      "cachegroup": "tallahassee-fl-usa",
      "parent": "mid-east",
      "secParent": "mid-west",
      "parents": [
        2,
        0
      ]
    }
  ]
}

If this is a bug fix, what versions of Traffic Control are affected?

• master (c8db585)

The following criteria are ALL met by this PR

• We are going to take out edge parentage validation anyway, so no tests
• Bugfix, no documentation necessary
• An update to CHANGELOG.md is not necessary
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[rawlinp]

This looks good, but we should add some more positive test cases to the TO API tests (the example in the description would be a good one).

[zrhoffman]

More positive test cases for the TO API tests added in 9709073e08, including @rob05c's.

[rawlinp]

TO unit tests and API tests pass, new positive test cases look good.