Iterate over given apps or INSTALLED_APPS and collect the content of each's
        settings file, which is expected to be in JSON format.

def _collect_settings(self, apps):
        """
        Iterate over given apps or INSTALLED_APPS and collect the content of each's
        settings file, which is expected to be in JSON format.
        """
        contents = {}
        if apps:
            for app in apps:
                if app not in settings.INSTALLED_APPS:
                    raise CommandError("Application '{0}' not in settings.INSTALLED_APPS".format(app))
        else:
            apps = settings.INSTALLED_APPS
        for app in apps:
            module = import_module(app)
            for module_dir in module.__path__:
                json_file = os.path.abspath(os.path.join(module_dir, self.json_file))
                if os.path.isfile(json_file):
                    with open(json_file, 'r') as fp:
                        contents[app] = json.load(fp)
        return contents