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)

This FIXME has been here for too long. We should either fix it if you can think of a better way to do it or remove it.