nway_conference == true was evaluating as true regardless if nway_conference was set to true or false, just as long as it had a value of any kind.
This also fixes it so that you can set nway_conference to false in the default settings and it will disable the network conference feature, previously once it was enabled on yealink phones it could not be disabled without manually changing it in the phone or a factory default.
Support enabling nway conference with Grandstream phones if nway_conference is set to true. If it is not true, it sets the conference URI back to blank to restore local conference
Fix config URL so that the phones don't get an invalid URL when $domain_name is unset. They would previously receive `/app/provision` with no domain/ip included, which makes the phone unable to provision again once they have been added to the Devices page without manually re-setting the config URL or factory defaulting the device.
The alternative solution is that the PHP code could always ensure that the $domain_name variable is set even if the device doesn't exist. My suggestion is to see if it can be set to the same domain/IP as the original provision request from this unconfigured device
