[SPARK-1517] Refactor release scripts to facilitate nightly publishing

This update contains some code changes to the release scripts that allow easier nightly publishing. I've been using these new scripts on Jenkins for cutting and publishing nightly snapshots for the last month or so, and it has been going well. I'd like to get them merged back upstream so this can be maintained by the community.

The main changes are:
1. Separates the release tagging from various build possibilities for an already tagged release (`release-tag.sh` and `release-build.sh`).
2. Allow for injecting credentials through the environment, including GPG keys. This is then paired with secure key injection in Jenkins.
3. Support for copying build results to a remote directory, and also "rotating" results, e.g. the ability to keep the last N copies of binary or doc builds.

I'm happy if anyone wants to take a look at this - it's not user facing but an internal utility used for generating releases.

Author: Patrick Wendell <patrick@databricks.com>

Closes #7411 from pwendell/release-script-updates and squashes the following commits:

74f9beb [Patrick Wendell] Moving maven build command to a variable
233ce85 [Patrick Wendell] [SPARK-1517] Refactor release scripts to facilitate nightly publishing

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