Did you successfully update with EPEL disabled? The post linked, regards problems with NSS being out of date, there were several other google results with similar info. We update daily with yum-cron, and I still had this come up this week with the EPEL signatures not matching, the problem did not affect my CentOS 7 systems, only my SL7 systems. That alone implied it was not an EPEL repo problem.
Anyway, after the yum clean, then updating with EPEL disabled, everything is fine, just as the post suggested.
