Deleted conductors are never removed

[Sandzwerg]

We retired some conductors together with the corresponding hardware nodes. The ironic nodes were deleted, the compute service were deleted and the conductors itself were stopped & deleted as well. However, the conductors are still listed in `openstack baremetal conductor list` as Alive = False. There does not seem to be a way to remove the conductors from there. Deleting from the D should work, but was not yet tested. Asking about in IRC it was suggested to have a periodic task that cleans up conductors that are dead for time X.

```
os baremetal conductor list | grep -v True
+-----------------------------------+-----------------+-------+
| Hostname | Conductor Group | Alive |
+-----------------------------------+-----------------+-------+
| ironic-conductor-255374503-5rev7 | | False |
| ironic-conductor-362656936-jpbus | | False |
| ironic-conductor-3244631988-vbfsl | | False |
| ironic-conductor-1030842398-ysauu | | False |
| ironic-conductor-1030842398-2mild | | False |
| ironic-conductor-2955838132-1dmb9 | | False |
| ironic-conductor-2955838132-kngz1 | | False |
| ironic-conductor-2955838132-jiksy | | False |
| ironic-conductor-2955838132-eggtu | | False |
| ironic-conductor-2955838132-v2df9 | | False |
| ironic-conductor-2955838132-l4hq4 | | False |
| ironic-conductor-2955838132-5619b | | False |
| ironic-conductor-2955838132-xkw9y | | False |
| ironic-conductor-2955838132-fpl14 | | False |
| ironic-conductor-ipmi | | False |
| ironic-conductor-ipmi-ipxe | | False |
| ironic-conductor-bm033 | bm033 | False |
| ironic-conductor-bm082 | bm082 | False |
| ironic-conductor-md026 | md026 | False |
+-----------------------------------+-----------------+-------+
```

These are all conductors that no longer exist. The first are all from some testing, I'm not sure where the ipmi ones are from, they might be ancient, but I can't remember every using them and the last three are all from hardware we decommissioned in the last ~6- month or so.