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.
Expose platform/version override options on `omnibus publish` CLI
Add features for new CLI flags in publish subcommand