Move plugins.d/README from the node to the broker

The /etc/openshift/plugins.d/ directory and the README file therein are
related to the broker, not the node.

This commit also modifies the spec file so that the README file is actually
shipped in the RPM.
Merge pull request #2062 from Miciah/move-plugins.d-README-from-node-to-broker

Merged by openshift-bot