HoS can't control number of map tasks for runtime skew join [Spark Branch]

Flags hive.skewjoin.mapjoin.map.tasks and hive.skewjoin.mapjoin.min.split are used to control the number of map tasks for the map join of runtime skew join. They work well for MR but have no effect for spark.
This makes runtime skew join less useful, i.e. we just end up with slow mappers instead of reducers.