Disable centos6 testing
Disable centos6 testing

Centos6 is no longer reasonable to expect to function with openstack,
and infra does not host a mirror for it.

Change-Id: I95ccf5840807fee73d6e78d596c82709e476bb3a

openstack/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder