Add option to allow list of users to access system config

Currently we allow access to system config for all users. Though we mask any sensitive information it is unnecessary to show it to all users. Can restrict the access to admins, but services like Hue use the server side config values to display UI accordingly (For eg: Server Timezone, whether SLA is configured, what kind of credentials are supported, etc). So it would be good to support restricting it to admins and a list of users.