enable-ha on lxd cluster doesn't evenly spread controllers

[Jeff Hillman]

juju 2.9.28

After substantiating an LXD cluster with 3 nodes, running `juju enable-ha` after bootstrap didn't evenly distribute the next 2 controllers

```
$ lxc cluster list
+---------+-------------------------+-----------------+--------------+----------------+-------------+--------+-------------------+
| NAME | URL | ROLES | ARCHITECTURE | FAILURE DOMAIN | DESCRIPTION | STATE | MESSAGE |
+---------+-------------------------+-----------------+--------------+----------------+-------------+--------+-------------------+
| repsold | https://172.16.7.3:8443 | database-leader | x86_64 | default | | ONLINE | Fully operational |
| | | database | | | | | |
+---------+-------------------------+-----------------+--------------+----------------+-------------+--------+-------------------+
| tebbutt | https://172.16.7.5:8443 | database | x86_64 | default | | ONLINE | Fully operational |
+---------+-------------------------+-----------------+--------------+----------------+-------------+--------+-------------------+
| townley | https://172.16.7.4:8443 | database | x86_64 | default | | ONLINE | Fully operational |
+---------+-------------------------+-----------------+--------------+----------------+-------------+--------+-------------------+

```

```
$ lxc list
+---------------+---------+---------------------+------+-----------+-----------+----------+
| NAME | STATE | IPV4 | IPV6 | TYPE | SNAPSHOTS | LOCATION |
+---------------+---------+---------------------+------+-----------+-----------+----------+
| juju-609cc2-0 | RUNNING | 172.16.8.195 (eth0) | | CONTAINER | 0 | repsold |
+---------------+---------+---------------------+------+-----------+-----------+----------+
| juju-609cc2-1 | RUNNING | 172.16.8.198 (eth0) | | CONTAINER | 0 | townley |
+---------------+---------+---------------------+------+-----------+-----------+----------+
| juju-609cc2-2 | RUNNING | 172.16.8.197 (eth0) | | CONTAINER | 0 | townley |
+---------------+---------+---------------------+------+-----------+-----------+----------+

```

```
$ juju status -m controller
Model Controller Cloud/Region Version SLA Timestamp
controller local localhost/localhost 2.9.28 unsupported 19:18:15Z

Machine State DNS Inst id Series AZ Message
0 started 172.16.8.195 juju-609cc2-0 focal Running
1 started 172.16.8.198 juju-609cc2-1 focal Running
2 started 172.16.8.197 juju-609cc2-2 focal Running

```

Expected behavior would be that the controllers would at least spread to separate cluster nodes, mimicking zone type behavior.