Update link for CentOS image
Update link for CentOS image

Changes in new image:
* update all packages
* fix shellshock vulnerability

Change-Id: I129aadf0519bcfdfc6c3988dd6a13ea0992c08e5

openstack/sahara-image-elements,crobby/sahara-image-elements,bigfootproject/sahara-image-elements,bigfootproject/sahara-image-elements,uladz/sahara-image-elements,openstack/sahara-image-elements,crobby/sahara-image-elements,uladz/sahara-image-elements