Convert sample object store configuration to YAML and support configuring inline

[natefoo]

You can now directly configure advanced object store configurations in object_store_config in galaxy.yml.

How to test the changes?

(Select all options that apply)

• This is a refactoring of components with existing test coverage.

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jdavcs]

Another improvement to our config! Looks good.

In general, I think, the logic of type "if config file exists - read from file, else - read from galaxy.yml" should live in galaxy.config. That way (a) all config-reading and processing logic would be in one place (and we'd have well-tested code that handles any and all such use cases); and (b) we'd know that whatever is in app.config.foo is the actual configuration of foo. But that's outside the scope of this PR, of course.