Propose additional IAM Role Actions

Depending on what settings you use for the amazon-chroot builder additional actions needs to be allowed in the IAM role for the aws instance.

This patch does not include an exhaustive list of all alternatives, but only the changes I needed to do to get stuff working for me.
builder/amazon/*,builder/openstack: properly scrub sensitive [GH-554]
communicator/ssh: buffer file on disk to read length [GH-561]
builder/virtualbox: allow disabling guest addition uploading
website: docs on disabling guest additions
command/fix: add fixer to auto set guest_additions_mode
update CHANGELOG
command/fix: add synopsis option to fixers

This will be used in help output and such later
Merge pull request #542 from phyrex1an/patch-1

website: propose additional IAM Role Actions
common: downloads can be HTTPS [GH-587]
packer: builder prepare can return warnings
packer/rpc: implement new warnings interfaces
packer/rpc: use packer.MockBuilder for tests
packer/rpc: test warnings with builders
packer/plugin: new Builder interface
builder/amazon/*: warnings
builder/all: update to warnings
command/validate: output warnings
command/build: get command passing
builder/virtualbox,vmware: warning if shutdown_command is not specified