nfd-master: ditch apihelper

Implement some of frequently used helper functions inpackage.

This patch also contains big changes to the nfd-master unit tests. Much
of this is about migrating from the mocked apihelper interface to fake
kubernetes client that provides a bit more apiserver'ish functionality.
At the same time there is quite a bit of renaming in the tests,
shortening and unifying naming and getting rid of the extensive usage of
"mock" everywhere.