version 0.1.8, handle multiple ubuntu versions and salt versions

* create and manage cluster with differing salt versions
* create and manage cluster with multiple ubuntu versions
* permit salt source to come from deb packages
* rearrange command-line options to support the above
* fix up ssh-to-minion and ssh-to-master to support new tag scheme
* fix up ssh-to-minion/master to not save container ips to known_hosts
  and not to bother with strict host key checking
* script 'generate_dockerfile.py' to generate dockerfile
  for each ubuntu version salt base image, from a template
* remove selinux hack from dockerfile builds (if you use an selinux
  enabled distro to host your docker containers, you will need
  to disable selinux in grub or via config)
* purge built images as a separate option to salt-cluster script
* script to remove base salt images
* script to download debs for the various versions of salt and
  their dependencies, for packages not in the ubuntu repo
* update the docs to reflect the above
* unpack salt package and configure without debian/postinst because
  recent versions of the package start salt automatically
* copy init.d scripts for salt from git repo, since the packages
  have symlinks to upstart which doesn't run on docker
* update puppet manifests to allow putting the salt config file in place
  separately from enabling the service; puppet < 3.2 will start
  (ensure => running) and then immediately restart (refresh from subscribe)
  the service, and salt has a zombie issue for some versions which
  the immediate restart sometimes trips
* pylint