Implement `fail` field

[jaypipes]

Currently, the assert field contains a set of assertions that are checked by gdt-kube. These assertions are retried until successful evaluation or a timeout (defaulting to 5s).

However, we want the ability to tell gdt-kube that if a certain condition is matched, that we should immediately fail the test. This would allow test authors to declare, for example, "if this Pod Ready condition goes to False, then fail the test and don't retry"

Possible example schema for the fail field:

tests:
 - kube.get: deployments/nginx
   fail:
     matches:
       spec:
         replicas: 1
   assert:
     matches:
       status:
         readyReplicas: 2

So, the above would attempt to find a status.readyReplicas field value of 2 for a default timeout of 5 seconds, but if at any time the spec.replicas field value is 1, then the test would immediately fail.