Rational:
* When the config is reloaded and the provider context is canceled, we need to
exit the current ZK `TargetProvider.Run` method as a new provider will be
instantiated.
* In case `Stop` is called on the `ZookeeperTreeCache`, the update/events
channel may not be closed as it is shared by multiple caches and would
thus be double closed.
* Stopping all `zookeeperTreeCacheNode`s on teardown ensures all associated
watcher go-routines will be closed eagerly rather than implicityly on
connection close events.
Allow namespace discovery to be more easily extended in the future by using a struct rather than just a list.
Rename fields for kubernetes namespace discovery
The staticcheck warns about testing.T usage in goroutines. Moving the
t.Fatal* calls to the main thread showed immediately that this is a good
practice, as one of the test setups didn't work.
