Use JSON files for the default configuration values

[avpaderno]

Some of the modules use code similar to the following one, to set the configuration values used by the module.

/**
 * Implements hook_install()
 */
function cron_example_install() {
  $config = config('cron_example.settings');
  $config->set('cron_example_interval', 60*60);
  $config->save();
}

Backdrop modules should use JSON files for the configuration default values and implement hook_config_info() to allow to Backdrop to remove those configuration values when the module is uninstalled.

At least the change should be done to show how to correctly use the Backdrop API. Example modules should not be installed on a production server; leaving some configuration values after the modules are uninstalled is probably a minor issue.

[avpaderno]

I am postponing this issue on #79. Before using a .json file for the configuration default values, values that should not be configuration values need to be replaced by state values.