DISTROS.md: drop fedora/24/cloud and update fedora/26/atomic
DISTROS.md: drop fedora/24/cloud and update fedora/26/atomic

I don't think anyone is using fedora/24/cloud anymore, so let's not
support it anymore.

Also update the description for fedora/26/atomic since it's now been
released and add the fedora/26/atomic/pungi stream which replaces it.

jlebon/redhat-ci,jlebon/redhat-ci