Fix setting ami_groups

Fixes spot instance cleanup.

website: Update some docs to point to Atlas instead of Vagrant Cloud

website: Tips for debugging a build.

#2346 updated to AMI that doesn't require accepting marketplace terms

builder/aws: fix adding launch permissions for created ami.

Updated to same ami as in main readme.md which is from community AMIs
and therefore doesn't require accepting the AWS marketplace terms
before using in Packer build.

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

Previously, groups were granted launch permissions by submitting a
ModifyImageAttribute request with the UserGroups parameter set
appropriately. This is no longer valid, as the LaunchPermission
parameter must be set.

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