Arthur/ssh check

Check meant to:
1. Check the status of SFTP
2.Check The response time 


@LeoCavaille 
You can refactor and make them clearer by using namedtuples:

```python
from collections import namedtuple
OPTIONS = [
    ('host', True, None, str),
    ('port', False, 22, int),
    ('username', True, None, str),
    ('password', False, None, str),
    ('private_key', False, None, str),
    ('sftp_check', False, True, bool),
]

Config = namedtuple('Config', [
            'host',
            'port',
            'username',
            'password',
            'private_key',
            'sftp_check',
        ]
        )
    def _load_conf(self, instance):
        params = []
        for option, required, default, expected_type in REQUIRED:
            value = instance.get(option)
            if required and not value or type(value) != expected_type :
                raise Exception("Please specify a valid {0}".format(option))

           
            if value is None or type(value) != expected_type:
                self.log.debug("Bad or missing value for {0} parameter. Using default".format(option))

            params.append(value)
        return Config._make(params)
```