Wrong additionalLocations in flavors lead to rejected Testruns

[dguendisch]

A flavor snippet like this:

- provider: gcp
  additionalLocations:
  - type: git
  allowPrivilegedContainers: false

Leads to a rendered Testrun that contains a location without any repo and revision and is therefore consequently rejected by the admission webhook.

admission webhook \"validate-testrun.tm.garden.cloud\" denied the request: [spec.locationSets[0].locations[84].repo: Required value: repo has to be defined for git TestDefinition locations, spec.locationSets[0].locations[84].revision: Required value: revision has to be defined for git TestDefinition locations]"

The flavor parsing should instead immediately reject meaningless additionalLocations fields (that have no repo and revision but just a type).