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 
I'm confused, don't we do the same thing here? https://github.com/opscode/omnibus/blob/v3.2.0/lib/omnibus/cli/base.rb#L87-L102