nova.conf contains some garbage options

[Takashi Kajinami]

The nova.conf file installed by the nova-common package contains a few unused options in the end.

I suspect these options are leftover of some old workarounds.

```
[cells]
enable = False

[os_region_name]
openstack =
```

This was found in Ubuntu 22.04 + UCA Bobcat.

# dpkg -l | grep nova
ii nova-api 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - API frontend
ii nova-common 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - common files
ii nova-compute 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - compute node base
ii nova-compute-kvm 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - compute node (KVM)
ii nova-compute-libvirt 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - compute node libvirt support
ii nova-conductor 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - conductor service
ii nova-novncproxy 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - NoVNC proxy
ii nova-scheduler 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute - virtual machine scheduler
ii python3-nova 3:28.0.0-0ubuntu1~cloud0 all OpenStack Compute Python 3 libraries
ii python3-novaclient 2:18.4.0-0ubuntu1~cloud0 all client library for OpenStack Compute API - 3.x

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.