Jenkinsfile: refactoring

Changes:
- all logical parts are moved to appropriate functions;
- stages are used only in main `buildImages()` and high-level
  functions to avoid nested stages usage to have a clean and
  comprehensive view of the job in Jenkins;
- use separate OSTree repositories to avoid confusion
  for parallel ostree tasks;
- use individual build statuses per image/tarball. With try/catch
  companion it allows to continue the build of deriviatives if some
  image fail to build;
- all artifacts deployed at stage 'upload' after the build;
- tests started only for successfully created images;
- allow to avoid the build `image` artifacts -- allow to create sysroot
  without the development image;
- for `Armhf` only minimal images and sysroot are created.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>