implement max_threads option for jetty

If you have a server with 256 GB RAM, the default jetty option for max_threads will not be enough.
The puppetdb.log will say:

```java.lang.IllegalStateException: Insufficient max threads in ThreadPool: max=100 < needed=121```

So we need the max_threads option to set the threads in the jetty.ini. 


Probably not worth providing an assumption here (ie. 120) as its misleading from the start and the default may change anyway. I'd just say it falls back to the PuppetDB default.