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provided by the centos-release-xen* packages are no longer valid, as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.

manifests/xen4centos/yumrepo/install.pp

@@ -31,6 +31,20 @@ define xen::xen4centos::yumrepo::install(
 	    "centos-release-xen${versiontag}":
 		ensure => installed;
 	}
+	# Point to a CentOS Vault mirror for older CentOS versions.
+	if (versioncmp($::operatingsystemrelease, 8) < 0)
+	{
+	    include pkgrepos::vars
+	    $repofile = "/etc/yum.repos.d/CentOS-Xen${versiontag}.repo"
+	    $vault_url = "${pkgrepos::vars::centos_vault_base}/${pkgrepos::vars::eol_rhel_last_version}"
+
+	    regexp_replace_lines {
+		'xen::xen4centos::yumrepo::centos-vault':
+		    file => $repofile,
+		    pattern => '^baseurl=https?://[^/]*/centos/.*/virt/(.*)',
+		    replacement => "baseurl=${vault_url}/virt/\\1";
+	    }
+	}
     }
     elsif ($ensure == 'absent')
     {