Use $settings instead of $config for content directory

Problem/Motivation

DCD uses $config['content_directory'] this is an unusual pattern for two reasons. The configuration does not contain a 'default_content_deploy' name space. Further this is this variable is not configurable via the interface and no initial value is available in /config/install.

Proposed resolution

The use of 'content_directory' is similar to the system public path. It is set in code and the audience is the developer/devops. I therefore propose to use the same pattern to set the content directory:

$settings['default_content_deploy_content_directory'] = '../content';

Remaining tasks

Perhaps add a deprecation warning for the admin.