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.

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

/cc @netshade

provisioner/chef-solo: chef environment support

website: Updated web site doc. for avail commands list

* Single quotes around chroot command cause it to fail.
* The chroot command also gets passed through the CommandWrapper template, so having sudo would clash with however the user wants to run it.
* Fix spelling mistake.

Added a test for the failure.

Most of the scanner code looks like it's a modified version of
bufio.ScanLines, so I changed it to use that but always add a line feed.

Packer was using the original file reader when uploading, instead of the UnixReader /cc @mitchellh