Added support to use templates for more fields in null-builder and amazon-builder

builder/amazon: support for enhanced networking

website: Added documentation page for environmental variables. #1130

website: Fixed Rackspace example

builder/vmware/esxi: checksum iso upload to not always upload

builder/virtualbox: Accept VirtualBox release candidate version

packer: Fixed typo in error message

website: Specifically call out the comma after `"builders": [...]`

Install gox during build time

post-processor/vagrant: hyper-v

Just so that we can just run `make` without installing gox separatelyy

Cleaned up Go formatting with gofmt.

While running through the getting-started examples, I ran into a strange
error:

	packer $ packer validate example.json
	Failed to parse template: Error in line 16, char 2: invalid character
	'"' after object key:value pair
	  "provisioners": [{
	packer $

The error was not immediately obvious, as this is my first time working
with Packer, and like many users this also happens to be my first time
working with JSON.

Solution:

I was missing a comma after the 'builders' configuration. I'm sure this
is a common problem, so let's specifically mention that in the document.

While running through the getting-started examples, I ran into a strange
error:

	packer $ packer validate example.json
	Failed to parse template: Error in line 16, char 2: invalid character
	'"' after object key:value pair
	  "provisioners": [{
	packer $

The error was not immediately obvious, as this is my first time working
with Packer, and like many users this also happens to be my first time
working with JSON.

Solution:

I was missing a comma after the 'builders' configuration. I'm sure this
is a common problem, so let's specifically mention that in the document.

As per https://github.com/mitchellh/packer/issues/1142, it's critical to add openstack_provider: rackspace in order to get anything to work reliably with Rackspace and Packer in 6.0.

Question: should the openstack_provider field now be marked as required?

[DOC] Changed to reflect the new httpS endpoint for Chef, since Chef 11.x

**New post processor:** `vagrant-cloud` - Push box files generated by
    vagrant post processor to Vagrant Cloud. [GH-1289]