Fixes #8316: Removing a host can create a huge transaction

Removing a host with some reports (say 200), and each of them with several log lines (10000) generates 2.000.000 DELETE sql statements in the same transaction.
More often than not, ActiveRecord won't be able to make such a big transaction, in fact it will likely eat all memory available (I tried on a machine 16GB RAM machine) and Foreman will stop working in that host. In the best scenario the DB will end up with a non-committed giant transaction.

I suggest deleting hosts and reports manually on host destroy to avoid this.
yes, this is what i had in mind too, in rails its simple:
```ruby
Log.where(:report_id => Report.recent)
```
will generate
```sql
SELECT "logs".* FROM "logs" WHERE "logs"."report_id" IN (SELECT "reports"."id" FROM "reports" WHERE (reported_at > '2014-11-17 11:36:53.150355') ORDER BY reported_at)
```

Also, there is a host expire reports method (when clicking on build it removes old reports & facts) maybe that should be unified.