Move Nagios to Checks.d

Not entirely done because I want to run some more tests, probably on staging, but creating the pull request in order to get some feedback.

With the migration to a checks.d check, some changes were introduced:

- Support for old metrics point: Because of this https://github.com/DataDog/dd-agent/blob/master/aggregator.py#L614, points too old are not supported by the Nagios checks.d while they were supported with the dogstream based check. I don't know if we want to override the default recent_point setting or not but I don't think it's necessary because I'm not sure exactly how far in the past can we submit points anyway

- Should we ensure configuration compatibility? (If no conf is available in nagios.yaml in conf.d, try to configure it with the content of datadog.conf)

Also can we verbify these settings ? Their meaning would be clearer.