provisioners: Import Docker provisioner code from Vocker
provisioners/docker: Simplify raising of Docker related errors
provisioner/docker: Add missing I18n
provisioners/docker: Clean up some leftovers from vocker
provisioners/docker: make sure version is correct type, fix Ruby warning
provisioners/docker: style nitpick on unless
provisioners/docker: allow images to be configured with images:
provisioners/docker: simplify run, output which are running
provisioners/docker: only allow 'args' special
provisioners/docker: style nitpicks
provisioners/docker: output for each image pulling
provisioners/docker: output version when installing
provisioners/docker: use proper UNSET_VALUE for no value
website/docs: document the Docker provisioner
Merge pull request #2549 from mitchellh/f-docker-provisioner

docker provisioner