Easy way to enable:

yum -y install yum-utils; \
cp -p /usr/share/doc/yum-utils-0.5/plugins/installonlyn/installonlyn.conf /etc/yum/pluginconf.d/; \
cp -p /usr/share/doc/yum-utils-0.5/plugins/installonlyn/installonlyn.py /usr/lib/yum-plugins/