NFS/VBox: remove dedicated cleanup action in favour of native routine.
Avoid udev rule masking and configure ifaces with sed instead.
systemctl wasn't a good idea
provisioners/docker: Only assign the `image` config in case it was not specified
website/docs: Add an example on how to run multiple containers based off the same image
website/docs: Add some links to Docker provisioner page
Merge pull request #2585 from mitchellh/2585-fix-docker-image-assignment

provisioner/docker: unable to run multiple docker containers based off the same image
Merge pull request #2591 from instilled/arch-netctl-fix

guests/arch: netctl fix
Merge branch 'nfs-sync-folder-cleanup' of https://github.com/emyl/vagrant into emyl-nfs-sync-folder-cleanup

Conflicts:
	plugins/providers/virtualbox/action.rb
synced_folders/nfs: style
scripts: bintray upload
core: Allow options to be passed down into SF cleanup
synced_folders/nfs: allow plugins to provide valid NFS ids
website/docs: affil links to book