Feature/unit tests (#53)

- Avoid relying on Clientset structure to call Kubernetes API functions.
While Clientset is a convinient "catch-all" abstraction for calling
REST API related to different Kubernetes objects, it's impossible to
mock. Replacing it wih the kubernetes.Interface would be quite
straightforward, but would require an exra level of mocked interfaces,
because of the versioning. Instead, a new interface is defined, which
contains only the objects we need of the pre-defined versions.

-  Move KubernetesClient to k8sutil package.
- Add more tests.