builder/openstack: If no image is generated, just return nil 

This mirrors what is done in other providers e.g. Amazon providers

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`