KAZOO-3037 Create maintenance routine for configuring SMTP config

https://2600hz.atlassian.net/browse/KAZOO-3037
Can you make this `sup notify_maintenance reload_smtp_configs` or something like that (and add the appropriate function in `notify_maintenance`); the code should just call `whapps_config:flush(<<"smtp_client">>)`. I want to move people away from flushing the whole cache when one doc is changed.