Tyr: change instance logger handling

instance logger is now copied from the 'instance' logger.

This way we can configure a custom formater for the instance (usefull
for the syslog for example).

logger conf example:
```json

LOGGER = {
    'version': 1,
    'disable_existing_loggers': True,
    'formatters': {
        'default': {
            'format': '[%(asctime)s] [%(levelname)5s] [%(process)5s]
[%(name)25s] %(message)s',
        },
        'instance': {
            'format': '%(name)s: [%(asctime)s] [%(levelname)5s]
[%(process)5s] %(message)s',
        }
    },
    'handlers': {
        'default': {
            'level': 'DEBUG',
            'class': 'logging.FileHandler',
            'filename': '/home/antoine/run/tyr/tyr.log',
            'formatter': 'default',
        },
        'instance': {
            'level': 'DEBUG',
            'class': 'logging.FileHandler',
            'filename': '/home/bob/run/tyr/%(name).log', --> for
FileHandler we can use the %(name) tag to output in a different file
            'formatter': 'instance',
        },
    },
    'loggers': {
        '': {
            'handlers': ['default'],
            'level': 'DEBUG',
        },
        'instance': {
            'handlers': ['instance'],
            'level': 'DEBUG',
        }
}
```
Tyr: change after review

this is way better, we can now specify a custom conf for one instance,
we just need to add a

'instance.bob': {...}

to have a custom logger for the 'bob' instance
Merge pull request #766 from antoine-de/tyr_log

Tyr: change instance logger handling