Add a base_name option to Debian packager

The name of the output file is currently inherited from the project. One could want to change the file name (say to `my-project-ubuntu_1.2.3-1.amd64.deb` but keep the name of the package identical ('my-project')'. For example, if Ineed to build two different using the Debian packager with different provisioining scripts depending on the build platform:

```ruby
package :deb do
  ...
  base_name "my-software-#{ohai['platform']}"
end
```