Add privacy flag

This adds a `privacy` flag to `metadata.rb` which will mark a cookbook as private (or not).  If you try to upload a cookbook marked private to Supermarket, and Supermarket is configured to enforce privacy, then the upload will fail.

Here is the original issue this was spawned from:  https://github.com/opscode/supermarket/issues/832
Here is the matching PR to Supermarket itself:  https://github.com/opscode/supermarket/pull/912

I tested this by doing `knife cookbook upload` of a cookbook with the `privacy` attribute set to true, against Chef Server 12.2.0.dev.0 running in a local VM and it succeeded.

Is there any further testing I should do or anything I've overlooked here?