Adding docs to #645 on the website

Setting `"ami_groups": ["all"]` will make the AMI publicly accessible.
Correcting website docs for this.

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