Validate config on startup

[luhring]

To the extent it's possible, the application should validate its received config as it starts up, and exit with a failure if there's a known problem with the config. The idea is to fail fast, and alert the user to a config problem as soon as possible — ideally, long before a Kubernetes object is actually received for validation.

For example, there is a known list of correct ResourceSelectorType values. If an unexpected value is used in the config, the app should error out and explain what the problem was.