allow users to provide a replacement command / extra args to the
ansible-local provisioner.

this is useful, for example, to unbuffer output when using the
docker builder, or to modify ansible behavior, like enabling
verbose mode, injecting variables, or changing paths.

Change the word `aws_access_key` to `aws_secret_key`.

website: update defaults for DO ID's

/cc @mmckeen

/cc @mmckeen - Did a refactor here, CC'd in case you're curious
or want to learn :)

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