Security issue, default key

[jmgurney]

Per the code at:

zigpy-cc/zigpy_cc/types.py

Line 110 in e75d1fc

zigpy.config.cv_key([1, 3, 5, 7, 9, 11, 13, 15, 0, 2, 4, 6, 8, 10, 12, 13])

If a key is not provided, an insecure key is used. Many applications do not provide a key, such as home assistant (at least at one point it did, they may have fixed it), which means that networks deployed w/ this default key are easy to hijack and control.

Please make it an error to not provide a secure key, or generate a secure key by default. A warning in the later case is likely useful as a restart will cause problems.