Fixes #6134 - Discovered hosts prefix can be configured

That's not correct. We must end up with both hostname set to `prefix+fact_name` and mac set to `fact_name`. I have to admit this was confusing:

```
app/models/host/discovered.rb
34:    fact_name = Setting[:discovery_fact] || 'macaddress'
```

Since the `discovery_fact` now defaults to `discovery_bootif` you should do this (it was a minor bug):

```
fact_name = Setting[:discovery_fact] || 'discovery_bootif'
....
h.mac = facts[fact_name].try(:downcase)
```