DISTROS.md: some more tweaks
DISTROS.md: some more tweaks

- Drop `fedora/25` images. All projects should have migrated by now.
- Drop `fedora/26/atomic/pungi`. It's no longer updated and wouldn't
  make sense to use.
- Add `fedora/26/cloud`. This was an oversight, it's definitely
  already supported.
- Add new `fedora/27/atomic/pungi` and `fedora/27/cloud` images.
- Update descriptions for these.

Closes: #76
Approved by: ashcrow

jlebon/redhat-ci,jlebon/redhat-ci