Split out kubernetes config file

[GerkinDev]

Hi!

For ease of maintenance and customization, it would be great to have multiple config files, one for each type of kubernetes log. In fact, in my situation, I have to override the parser for audit log (see #519), but still rely on default config for the others.

Would you welcome such refactor?
Moreover, this could be a non-breaking change I guess, because this would be feasible to change the main kubernetes.conf file as an includer of each sub-file. So, if it is replaced, new includes would be omitted.

In a longer sight, this would allow more souple configuration from env. Something like this in kubernetes.conf

include "kubernetes-audit-#{ENV['FLUENT_KUBERNETES_AUDIT_MODE'] ||"legacy"}.conf

[repeatedly]

Looks good for me. Could you send a patch?

[GerkinDev]

I'll soon open a pr with this ;)