We still prefer the IPv4 address, but if there is an IPv6 address and no IPv4 address, use IPv6.

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`