Add explicit setting for WPA2 enterprise

[walkerdanny]

Currently the process of updating the WiFi config requires setting wifi_wpa2ent_username to None to disable it, which is both confusing, and throws a KeyError if a user tries to do it twice.

It would be good to have this as an explicit separate setting, like wifi_wpa2ent_enable that can be set to avoid the confusion, and remove problems such as users setting the username to the string "None" etc.