Add exec to pod method in KubeClient

[name212]

In kube:

• add to KubeClient Exec method for run command in pod container.
  It needs for hide implementation details, because exec in pod client-go method require rest config.
  We can get it only for internal client, not interface
• add NewFakeKubeProviderWithExec and NewFakeKubeProviderInitializerWithPodExec for create fake client with fake executor. For testing purposes you can implement your own PodCommandExecutor and pass it.
• fix potential panic with initialization for ErrorKubernetesClient