Make test_rdo_upgrade run from a minimal CentOS install (and fix some packstack annoyances)

There are two sets of changes here:

- Install basic requirements (wget and dbus) if they don't exist.
- Set up ssh authentication for root@localhost since packstack doesn't seem to get this right.

With these changes in place, my RHEL and CentOS systems now fail in the same way (they get as far as trying to launch an instance on havana).