Validate CR during the kubectl apply/create using CRD validation feature

[olegchorny]

https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/#validation

[TenSt]

Partially implemented by #37.
Will create new issue to implement advanced validation for TestRun and CronTest.

[TenSt]

Created validation is not working fine with TestRun and CronTest - removing it, will need to create new one.