[setup] Allow specification of major / major-minor version

[strideynet]

As it stands, the entire version has to be provided with setup.

It would be cool if we could automatically pick the latest that matches the major or major-minor pair that the user has specified.

This is as a result of feedback from someone trying to set up Teleport and only providing the major version. Perhaps for now we ought to add more validation to detect this and warn.