Implement a new feature source named 'local' whose only purpose is to
run feature source hooks found under
/etc/kubernetes/node-feature-discovery/source.d/ It tries to execute all
files found under the directory, in alphabetical order.
This feature source provides users a mechanism to implement custom
feature sources in a pluggable way, without modifying nfd source code or
Docker images.
The hooks are supposed to print all discovered features in stdout, one
feature per line. The output in stdout is used in the node label as is.
Full node label name will have the following format:
feature.node.kubernetes.io/<hook name>-<feature name>
Stderr from the hooks is propagated to nfd log.
