Set 'tez' as default value in hive.execution.engine

Maybe this is not the first ticket addressing this, please link if it's a duplicate.
We need to set this to 'tez' to reflect that we have deprecated 'mr':
https://github.com/apache/hive/blob/bd16e0098916aa5fc2dede99492c6a240b51e677/common/src/java/org/apache/hadoop/hive/conf/HiveConf.java#L4567

I'm expecting lots of UT failures because of this, as we're still running those on mr (which might be fine where the actual unit test is not closely related to the execution engine), so we'll see what to do.