Add S3-backed config storage

[zeotuan]

What is this PR for?

A few sentences describing the overall goals of the pull request's commits.
First time? Check out the contributing guide - https://zeppelin.apache.org/contribution/contributions.html

What type of PR is it?

Bug Fix
Improvement
Feature
Documentation
Hot Fix
Refactoring
Please leave your type of PR only

Todos

• - Task

What is the Jira issue?

• Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN/
• Put link here, and add [ZEPPELIN-Jira number] in PR title, eg. [ZEPPELIN-533]

How should this be tested?

• Strongly recommended: add automated unit tests for any new or changed behavior
• Outline any manual steps to test the PR here.

Screenshots (if appropriate)

Questions:

• Does the license files need to update?
• Is there breaking changes for older versions?
• Does this needs documentation?

[jongyoul]

Hi @zeotuan, thanks for the PR. Could you do two things?

1. Prefix the title with [MINOR] since the actual code change is small (a warning log) and the bulk of this PR is documentation/templates around already-working S3A support via FileSystemConfigStorage.
2. The PR description is still the empty template — could you fill it in with a brief summary of what this PR does and why?

Thanks!