allow for "clean_repos" yum provisioner option

You can copy the yum cache files into the chroot then specify which repo to clean:

provision_config_files:
  - /etc/resolv.conf
  - /var/cache/yum/addons
  - /var/cache/yum/base
  - /var/cache/yum/extras
  - /var/cache/yum/updates

clean_repos:
  - my-special-repo

This will allow yum to use the metadata cached on the master (outside the chroot) and then just refresh the metadata for specific repos that you frequenly publish rpms to.