Remove Yum metadata if the repository descriptions change

Repository description changes may imply:

* New repositories, in which case their metadata has to be downloaded anyways.
* New URLs for existing repositories, in which case the metadata we hold is invalid and has to be redownloaded.  Failing to do so may stall Yum, if the metadata in the new location is older than our caches (f.i, when rolling back a snapshot).

This fixes the bug reported by Denis Pugnere in the mailing list.