1138475 - yum distributor now always includes "description" element for errata

This resolves an issue where pulp could not sync an EPEL5 repo that had been
published by pulp. The upstream repository included an empty description
element, so the distributor was changed to produce XML with a similar empty
element.

https://bugzilla.redhat.com/show_bug.cgi?id=1138475
I do have a soft, human brain…