mod_callcenter: Add ability to unload/reload/load a queue setting (You still…

mod_callcenter: Add ability to unload/reload/load a queue setting (You still need to reloadxml before).
Note that joining a queue will check for it in the config and load it on the fly... I've used the same system as in mod_voicemail.  Not sure if we should allow this, but just comment it out of the config before unload and it wont be available anymore