Add support for overriding publish platform/version

This is useful in situations where packages that are build on one platform/version are tested and released on 1..n other platforms/versions. For example, one might build on Ubuntu 10.04 and test/publish on Ubuntu 
11.04, 12.04 and Debian 7.

/cc @opscode/release-engineers 
So I think warn is appropriate since the message warns about potential surprising behavior. I also think it needs to be shown by default even if a `-l info` was given.