test/upgrade: Test multiple upgrade path in Ubuntu

Originally, we tested only precise->saucy, through non-LTS releases.

Now, we test quantal->saucy (direct, as raring was marked unsupported
before quantal), raring->saucy->trusty, and LTS upgrade precise->trusty.

precise->quantal is broken (upstream problem due to, at least,
libxi6 needing to be downgraded), but I don't see why anyone would
still want to do that now that trusty has been released.

Finally, the logic tests if all release are in SUPPORTED_RELEASES
or are unsupported, to make sure the upgrade path is tested when
required.