Don't block Nebula startup on the presence of routines on non-Linux OS

[johnmaguire]

Oftentimes users have a config.yml template that they apply to many machines. Unfortunately, some OS do not support all config options. As a simple example, tun.name is ignored on macOS. However, routines is a hard failure on Windows. routines has performance implications but doesn't affect the security or availability (except perhaps in extreme cases of an overloaded CPU) so it's probably safe to simply emit a warning.