enable "auth_schemes" "vencrypt" to enable cert based auth between novncproxy and qemu

[Sven Kieske]

config reference:

https://docs.openstack.org/nova/latest/configuration/config.html#vnc.auth_schemes

to prevent direct access to the qemu-console cert based auth should be used.

Todo: find out which certs to use for this, maybe something already present can be reused here.

[vnc]vencrypt_client_key, [vnc]vencrypt_client_cert: must also be set

a user reported this as a possible (security) bug via IRC.

currently it is possible to connect to any vnc session with any vnc client by just pointing the client at the port/ip combination.

depending on your point of view this might be viewed as a security bug, but as the information is already publicly available both in code as in the IRC chatlogs I choose not to open a private bug for this.