builder/amazon: support launching in VPC

adds `vpc_id` and `subnet_id` to builder config

depends on https://github.com/mitchellh/goamz/pull/4

/cc @sit - Found another edge case

/cc @timsutton - This is a bit more flexible.

builder/vmware: Remove ISO from ide1 in stepCleanVMX

builder/v*: Include path to output dir in error

/cc @sit

/cc @sit

Preserve suffix of files put in the cache.

builder/vmware, builder/virtualbox: add special keys for backspace, delete and F1-F12

This reverts commit 688be43811007d33b9c70dec93b43782a4f71c06; instead
of copying the ISO (which can be huge), we have adjusted the cache path
to include the extension of the source file.

Conflicts:

	builder/virtualbox/step_download_iso.go

This allows us to hand cache paths to any programs that may want to
interpret file extensions in order to behave differently. For
example, VirtualBox may want ISO images to end with .iso.

/cc @markpeek - The main thing is that the style for variables/methods
in Go is to use CamelCase (with the first letter generally downcased
unless an exported thing).

communicator/ssh: check scp codes and report errors

packer: builder name should be removed from rawConfig

This prevents the builder from rejecting the name (which it doesn't
know about) when validating the config.

The name isn't actually part of the builder config, so it should be
removed during parsing.