Fix: Update CentOS 7 mirrorlist to vault

Fix: Update CentOS 7 mirrorlist to vault

With CentOS 7 being EOL the mirrorlist URLs are archived
and moved to CentOS vault.

This impacts Jenkins minions configuration required for
Sigul signing.

As a workaround force vault use in yum repos config
till this is resolved and new version of sigul client/server
is available to use on the infrastructure.

https://forums.centos.org/viewtopic.php?t=78708&start=30
https://forums.centos.org/viewtopic.php?t=80698

Issue: RELENG-5419
Change-Id: Ic193107a29aa83a2ee3c2cc356ca624522cc94f9
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>