Hi, All.

There was a discussion on publishing artifacts built with Hadoop 3 .
But, we are still publishing with Hadoop 2.7.3 and `3.0-preview` will be the same because we didn't change anything yet.

Technically, we need to change two places for publishing.

1. Jenkins Snapshot Publishing
    https://amplab.cs.berkeley.edu/jenkins/view/Spark%20Packaging/job/spark-master-maven-snapshots/

2. Release Snapshot/Release Publishing
    https://github.com/apache/spark/blob/master/dev/create-release/release-build.sh

To minimize the change, we need to switch our default Hadoop profile.

Currently, the default is `hadoop-2.7 (2.7.4)` profile and `hadoop-3.2 (3.2.0)` is optional.
We had better use `hadoop-3.2` profile by default and `hadoop-2.7` optionally.

Note that this means we use Hive 2.3.6 by default. Only `hadoop-2.7` distribution will use `Hive 1.2.1` like Apache Spark 2.4.x.

Bests,
Dongjoon.