Exclude downburst VMs
Exclude downburst VMs

Their cloud images have SELinux disabled; exclude them until we can
change that.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

dreamhost/teuthology,caibo2014/teuthology,ktdreyer/teuthology,ktdreyer/teuthology,robbat2/teuthology,SUSE/teuthology,michaelsevilla/teuthology,robbat2/teuthology,SUSE/teuthology,dmick/teuthology,dmick/teuthology,ivotron/teuthology,ceph/teuthology,dreamhost/teuthology,ceph/teuthology,SUSE/teuthology,michaelsevilla/teuthology,dmick/teuthology,caibo2014/teuthology,ivotron/teuthology