SPARK-5542: Decouple publishing, packaging, and tagging in release script

These are some changes to the build script to allow parts of it to be run independently. This has already been tested during the 1.2.1 release cycle.

Author: Patrick Wendell <patrick@databricks.com>
Author: Patrick Wendell <pwendell@gmail.com>

Closes #4319 from pwendell/release-updates and squashes the following commits:

dfe7ed9 [Patrick Wendell] Merge remote-tracking branch 'apache/master' into release-updates
478b072 [Patrick Wendell] Merge remote-tracking branch 'apache/master' into release-updates
126dd0c [Patrick Wendell] Allow decoupling Maven publishing from cutting release