fix typo in docs for saltmasterless provisioner. 

website/docs: note shutdown to run as root, why

/cc @mwhooker

provisioner/shell: fix EOF detection

Fixes #507

The scanUnixLine function was erroneously returning empty lines when EOF
was reached.
This commit adds a test for the problem and changes the scanUnixLine
function to elide the trailing \n if no content was read.

Also added a note about shutdown_command that bit me.

post-processor/vagrant: support instance store AMIs [GH-503]

builder/virtualbox: sleep after shutdown before export to avoid session locks

builder/amzon: fix race condition after launching EC2 instance

common/config: config filter function [GH-521]

Fixes #521

It is possible for an instance to not immediately exist after it is
launched. Previously, InstanceStateRefreshFunc would crash if this race
condition were realized.

This change takes the exact same approach of the function above,
AMIStateRefreshFunc, treating 'InvalidInstanceID.NotFound' as if there
were an empty result.

builder/digitalocean: Scrub config before logging

common: An SSH timeout should be treated as an error