Dogen

Changes
- Fixed dist-git plugin which failed when there were no sources to upload
- Updated dist-git plugin to search for new style labels

Changes
- Fixed 49 - All discovered plugins are now invoked and are responsible to check if they can perform (all requirements are met)
- Updated template to use new style labels

Enhancements
- All plugins now have access to command line arguments and also can register its own options via `Plugin.inject_args()` method
- Dist-git plugin is now used only if `--enable-dist-git` option is passed to dogen

Descriptor format changes
- `openshift` key was removed, use newly added `user` instead
- `hash` was renamed to `md5sum`

Other changes

`--scripts` CLI parameter was renamed to `--scripts-url`

Enhancements
- Support for CCT: https://github.com/containers-tools/cct
- Default user is now `root`
- Added schema validation using kwalify
- Extended tests
- Support for runtime changes with CCT, 26
- A bit more documentation, see https://github.com/jboss-dockerfiles/dogen/tree/master/docs