End need to specify or change distribution and dist_name in most cases.

Seems to work right now for Ubuntu or CentOS. SL and Debian would still need to specify these, but they would anyway so this is not a step backwards (anyone have a test environment to fill out the rest of _determine_distribution)?

Submitting this as a pull request because the changeset that added the dist_name check broke a lot of stuff I didn't totally understand, is this going to break to anything?