This is an annoying gotcha that I keep meaning to document. When installing collectd and rrdtool via yum on Centos 6/7, I always miss out the rrdtool lib. This manifests as
Starting collectd: Could not find plugin rrdtool.
when restarting the services.
Thankfully it is easy to fix. Run
yum install collectd-rrdtool
and restart the collectd service and it should start without complaint.