Use CentOS Vault for old CentOS versions.

For older, retired, CentOS versions, the URLs in the Yum repo files
provided by the centos-release-xen* packages are no longer valid, as
they point to old locations before that CentOS version was retired.
To handle this, we here let the xen::xen4centos::yumrepo::install
class edit the repo file to point to NSC's CentOS Vault mirror, Vault
being where CentOS publishes old version of their releases.