Allow specifying project for source images in GCE

Within GCE, images may be shared across projects. Prior to this
commit, there was no way to inform the GCE builder that a source
image belonged to a specific project. This adds an optional
'source_image_project_id' key to the GCE builder config.

fix panic when vagrant cloud response fails

Fix typo in docker builder docs

add disk size to google compute

parallels-iso: ISO not removed from VM after install [GH-1338]

Also remove floppy and prl-tools ISO if applicable.

website: typo in vagrant cloud 

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