VNC console support for Ironic driver

As part of the initiative to offer graphical console access to baremetal nodes, this change suggests making use of Ironic's graphical console capabilities within Nova, through a virt driver's ``get_vnc_console`` method.

The ironic virt driver would call Ironic's ``get-graphical-console`` and return a ``ctype.ConsoleVNC`` with the required connection information. Additionally, we would add a configuration option to allow the driver to auto-start the console whenever requested. This would allow a behavior similar to libvirt, where the console is always enabled.

To allow backward compatibility, ``get_vnc_console`` should raise ``ConsoleNotFoundForInstance`` in the following cases:

  * Ironic is running a version prior to the introduction of the graphical console calls

  * ``get-graphical-console`` raised an exception because the console was not running

  * ``set-graphical-console-state`` raised an exception because the node is using a driver that does not support the graphical console

Gerrit topic: https://review.openstack.org/#q,topic:ironic-vnc-console,n,z

Addressed by: https://review.openstack.org/398270
    DNM PoC Enable VNC console access to Ironic nodes

[20230116 bauzas] Spec approved for 2023.1 cycle https://review.opendev.org/c/openstack/nova-specs/+/863773/

[20230307 bauzas] Deferred as implementation not merged in 2023.1

Work items:
Implement get_vnc_console (incl. integration tests): TODO
Allow autostart if console is down: TODO