This implements more robust interrupt handling by propagating cancels
throughout more core Packer components.

/cc @mwhooker - I moved the processing up into Prepare so that any
errors will be shown during a validate pass. Also, I escape some stuff
in keys. Tests moved as well.

provisioner/chef-solo: template expansion of `json` 

website: openstack s/droplet/server/

website: Fix incorrect config key for Chef staging directory

Fixes #362