Upgrades, fix rollback for bootstrap engine

* changed rollback strategy for bootstrap
  upgrade engine, in previous strategy we
  made symlink from previous version of
  bootstrap to /var/www/nailgun/bootstrap
  it doesn't work because in 5.0 version
  cobbler container mounts /var/www/nailgun
  directory as /repo, as result symlinks
  which we make on the host system are
  broken inside of container.

  Instead of symlink command we rename
  previous version to /var/www/nailgun/bootstrap

* use lexists instead of exists because
  exists returns False on broken symlinks

* fixed docker engine rollback, there
  was wrong definition of previous version
  of fuel

Change-Id: Id0b50835410e005fde76f421981b4616211ddcbf
Closes-bug: #1348166