fix(instances): a new VM and each clone get a VNC password of their own

Since the VM create became a job (6f0c7300) every new VM was defined with
no VNC password, and a clone copied its source's password (or its lack
of one) exactly: every clone of a template shared it. Computes listen
for VNC on all their addresses (dev/libvirt-bootstrap.sh), so anyone who
could reach a compute's VNC ports opened any of these consoles directly,
without the panel; the README's firewall rule was the only guard.

The create job asks for create.py's random 8-character password again,
and a clone's VNC graphics get a new random one. The console page sends
the password to a user with passwordless_console, which the user create
form now ticks by default next to change_password; a user without it is
asked for the password, as the permission intends. Existing VMs keep
their settings.

Live create, clone and VM page tests pass on a TCP compute.

README.md

@@ -769,6 +769,7 @@ What an update from an older version changes for operators and users:
 - Failed logins lock a user (django-axes). The limit and lock time are on the Settings page; unlock with `python3 manage.py axes_reset_username <username>`.
 - Changing one's own password is the "Can change password" permission (per user or group; new users have it); `SHOW_PROFILE_EDIT_PASSWORD` is gone.
 - The admin's first-login password change survives losing `data/admin_password` (migration `accounts.0007`); run `migrate` as a user who can read that file.
+- A new VM and each clone get a random VNC password of their own (clones no longer share their source's). The console page sends it for a user with **Open a console without the VM's console password**, which a new user has unless unticked; others are asked for it. Existing VMs keep theirs.
 - An `admin` account that still has the password `admin` (the first user of versions up to v2.0.0) must change it at its next login, and its API tokens are refused until then (migration `accounts.0013`).
 - A `settings.py` kept from an older version must match the template (apps, middleware, the SQLite `OPTIONS`, the REST framework, knox, axes and password settings): rebuild it from the template and carry your own values over, as step 2 of the update does.
 - The VNC console connects to `/novncd/` on the panel's own address through nginx, and the serial console to `/socket.io/`: they work on HTTPS and behind another proxy without settings. `WS_PUBLIC_PORT`, `WS_PUBLIC_HOST`, `WS_PUBLIC_PATH`, `WS_CERT` and `SOCKETIO_PUBLIC_HOST`/`_PORT`/`_PATH` are gone; remove them from `docker-compose.override.yml`, `/etc/default/webvirtcloud` or a kept `settings.py`, set `WS_HOST = "127.0.0.1"` in a kept `settings.py`, and close port 6080 in the firewall.