Code review comment for ~pzakha/cloud-init:userdata

Thanks for updating the branch. I didn't see a unittest for the change. Let me know if you need help there.

I definitely would like to see us document this behavior. At a min, this branch can update:

 cloudinit/config/cc_ssh.py

The large block comment at the top is in RTD format, please add the allow_public_keys field, default value and describe what it does.

And I think the best location to mention disabling user-data is in:

 doc/rtd/topics/boot.rst

under the Network section. It mentions that this stage consumes all user-data.
Here you could add a

**Note**: Cloud-init system config (/etc/cloud/cloud.cfg.d/*) may disable consumption of
user-data via the following key: allow_userdata , etc

And document the default value and what it does.