Shell for the octavia user should be set to nologin

By default, the shell for octavia user is set to be /bin/sh:

```
# grep octavia /etc/passwd
octavia:x:116:124::/var/lib/octavia:/bin/sh
```

However, the CIS hardening rule "Ensure system accounts are secured" requires system accounts to be secured and the shell set to nologin.

As a workaround, you can run the following on octavia units:
```
# usermod -s "$(which nologin)" octavia
```