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