Prevent yum's repomd.xml from being cached by proxies

repomd.xml is always present and links to uniquely named files each time we
generate repos, so when transparent proxies cache it, but not the other files
then the repodata is out of sync for clients.

Disable all caching of repomd.xml alone so clients should see immediately when
the repo changes without affecting caching of other files.  (No explicit cache
controls headers are added by default.)