Update GCE source_image in the docs

builder/googlecompute: rename "Gsutil" to "Gcloud" now that we're updating "gcloud" and not "gsutil".

Also renamed files accordingly.

builder/googlecompute: added support for all standard VM images

[GCE] google compute builder is broken, gsutil update fails in newer images

Do not re-add floppy disk files to VMX

This commit fixes errors like this in the vsphere post-processor when using floppy files in the builder step:
Error: File (/var/folders/zl/57c1vmr532z_ryf1scw53_b9ycmxh7/T/packer964492999) could not be found

The configure VMX step re-adds the floppy files, so we need to configure the VMX and _then_ clean the VMX in that order.

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.

Also fixed the error message to be clear that the image was not found in any of
the projects that we attempted to search, rather than keep the error message
from the last project, which may be confusing to users.