Support generation in pwd for Dockerfiles

• Dockerfile_distro.generate_dockerfiles goes into the current
  directory instead with each Dockerfile suffixed with the release
  name. There is a new generate_dockerfiles_in_directories
  for the old behaviour.
• Move slow ARM distribution out of the default distro list into
  Dockerfile_distro.slow_distros.
• Add optional ?pin argument to dockerfile_distro generation
  to make it easier to customise version of packages installed.