Change docs usage of aws.ami to amazon.ami to be consistent with terraform

builder/amazon-instance: Don't specify empty Virtualization Type

InvalidBlockDeviceMapping

Fix failing AMI snapshot tagging when copying to other regions.

Fix for tag creation when creating new ec2 instance.

- Changed config from pointer to value to fix crash
- Removed acceptance flag from compress tests since they would have
caught this

builder/amazon: Fix issue with sharing AMIs with ami_users

clean up character encodings

website: Fix DigitalOcean section of getting started docs.

This updates the example for adding DigitalOcean
as a provider in the beginning tutorial. It looks
like it hadn't been updated in some time, and
was using old settings. I've updated it and
confirmed it works.

helper/communicator: support disabling SSH agent

builder/amazon-chroot: add mount_options configuration option

builder/qemu: default acceleration to tcg on Windows [GH-2284]