common: set user agent in downloader

builder/virtualbox,vmware,qemu: add more special scancodes

provisioner/ansible: upload playbooks correctly

website: fix typo in qemu documentation

`Provision` calls uploadFile on each path in playbook_paths, but
playbook_paths can only contain directories, per bf7530ca28.

This changes `Provision` to call uploadDir instead.

Updating the googlecompute builder documentation.

Update post-processor.html.markdown

Fixed some typos in the interface{} mentions.

Update virtualbox-iso.html.markdown

Changed type to virtualbox-iso

Fixes #806 Build fails if GOPATH has multiple paths

website: includes information about "provider"

none

website: really fix puppet provisioner cmd docs

builder/docker: include user vars for docker configuration