Packer fails when missing node_name

[bklang]

This is following up on #1100.

The field node_name is described as "optional", though the build fails without it because knife is unable to remove the chef client from the chef server after a successful configuration run. This results in the VM being destroyed.

I can think of a couple of ways to address this, but I'm not sure which you prefer:

• Set the default node_name to localhost, which appears to be what chef-client (or perhaps just the guest OS) defaults to
• Alternatively (and this is probably safer) make node_name a required attribute

[mitchellh]

I think this is a dup of #1295. This should get fixed with that.