Fix platform_version on Debian.

For review: @kvs @phlipper 

Debian's `node[:platform_version]` is based on `/etc/debian_version`, which is kinda useless because it tends to be a codename (which opscode's site doesn't support), and if you install sid's `base_files` (which can happen a lot accidentally), you end up with something like `jesse/sid` where jesse is the upcoming unreleased next stable version.

This updates the omnibus updater to use the first number in the `node[:lsb][:release]` package.  There's rarely any reason to upgrade the `lsb-release` package unless you're legitimately doing a whole system upgrade, so it continues to report the proper base version (6.0.8, shortened to 6).