Add a variable to configure repos disabled in base CentOS image
Add a variable to configure repos disabled in base CentOS image

[1] disabled CentOS Ceph Nautilus repo for Stein.
It turned out to be a problem for TripleO ([2])
which does not use upstream Ceph repo and actually
relies on CentOS Ceph Nautilus repo.
We did not leave the option to override our customization.
This patch adds a customizable variable to base CentOS image:
centos_yum_repos_to_disable

[1] https://review.opendev.org/651601
[2] https://review.opendev.org/664244

Change-Id: I867ca75cafe853b206beae2f81dfb79b62d05557
Signed-off-by: Radosław Piliszek <8759e5988b55949fdf66960d1c0fc9651a951f3b@gmail.com>

stackforge/kolla,stackforge/kolla,rahulunair/kolla,openstack/kolla,rahulunair/kolla,openstack/kolla,stackforge/kolla