common: StepDownload can force an extension

Update AWS EBS builder to fix invalid params

Revert "Changing --region to --location"

This reverts commit f40fd36c.

According to the documentation below, `--region` is the current flag. If you're
using an older version of the tools that use `--location`, you can customize the
commands in your Packer config with `bundle_vol_command` and `bundle_upload_command`

- http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/CLTRG-ami-upload-bundle.html
- https://www.packer.io/docs/builders/amazon-instance.html

Add 1/10th second delay between key events to VNC

virtualbox,vmware: http server should listen on IPv4

Updates to Into: Build An Image docs

website: Fix statement in machine-readable 'ui' type docs.

post-processor/atlas: fix index out of range panic

Tests were failing incorrectly when OpenStack environment variables were...

Fix digitalocean provider for private images [fixes mitchellh/packer#1792]

virtualbox/common: remove devices should delete floppy controller

builder/google: don't hardcode SSH timeout [GH-1781]

GH-1879

builder/aws: Add pre validate step, to validate things before building.

Fixes 'unknown configuration key' errors for 'only' and 'except'