test/upgrade: Test multiple upgrade path in Ubuntu

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

Now, we test quantal->saucy->trusty (this skips raring, as it was
EOL before quantal), raring->saucy, 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 test only runs if `-R` is not specified on the
command line: we do not want to check against SUPPORTED_RELEASES
as this test intentionally tries out releases that are marked as
unsupported.
Merge pull request #787 from drinkcat/upgrade-test

test/upgrade: Test multiple upgrade path in Ubuntu
Revert "Temporary workaround for #777 while we investigate googlesource.com"

This reverts commit a003c45ebfe966383b2715e234e12e319cba140c.
installer: Make sure there is at least 1 target to be installed.

Ancient chroots did not provide a target list, this serves as a reminder.

Fixes #749 and #788.