Add dependency on mysql jdbc connector for hive

Using hive with a mysql backend breaks if the mysql jdbc connector is not installed. We used to include this, but have since removed it. If it's not available, hive fails with a fairly opaque error (by throwing java.lang.reflect.InvocationTargetException). For this reason, we should at least add a Recommends: on the distro-provided mysql jdbc package, as well as adding a very loud note to the hive upgrade documentation.