Add service check for gunicorn

Alerts critical if no process with given name is found or if no workers are present (idle or running). 
OK if workers are running or idle.

Description for UI: https://github.com/DataDog/dogweb/pull/5844
why not clean this up a bit?
```
status = CRIT if working == 0 and idle == 0 else OK
 self.service_check(self.SVC_NAME, status, tags=['app:'+ proc_name], message=msg)
```

it makes the intent clearer. always submit a value, just with a different status and less duplicated code.