Add SSL certificate service check

Now the `_check` function from NetworkCheck can return a single service check or a list of service checks.
You can assume that init_config will always be a dictionary https://github.com/DataDog/dd-agent/blob/master/config.py#L776-L781

It will make your code simpler:
```python
self.ca_certs = init_config.get('ca_certs', LINUX_CA_CERTS)
```