CumulusLogger: only create new handler if one is not present

This fixes the test added in the previous commit

Before this change, multiple calls to `CumulusLogger(name)` would construct a
`CumulusLogger` instance with the same underlying `Logger` instance, but each
construction would also create and attach a new handler, resulting in duplicate
messages, e.g.:

```
> python
>>> from cumulus_logger import CumulusLogger
>>> logger = CumulusLogger('test')
>>> logger.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:18:09.315309", "level": "info"}
>>>
>>> logger2 = CumulusLogger('test')
>>> logger2.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:18:16.845070", "level": "info"}
{"message": "hello there", "timestamp": "2021-12-21T16:18:16.845070", "level": "info"}
>>>
>>> logger.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:18:18.165564", "level": "info"}
{"message": "hello there", "timestamp": "2021-12-21T16:18:18.165564", "level": "info"}
```

See also the screenshots on
ghrcdaac/dmrpp-file-generator-docker#25

With this change, a new handler is only constructed if the underlying `Logger`
does not already have any handlers, preventing messages from being handled more
than once:

```
> python
>>> from cumulus_logger import CumulusLogger
>>> logger  = CumulusLogger('test')
>>> logger.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:33:56.735453", "level": "info"}
>>>
>>> logger2 = CumulusLogger('test')
>>> logger2.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:34:03.887972", "level": "info"}
>>>
>>> logger.info('hello there')
{"message": "hello there", "timestamp": "2021-12-21T16:34:07.808415", "level": "info"}
```

cumulus_logger.py

@@ -88,15 +88,18 @@ def _get_exception_message(**kwargs):
 class CumulusLogger:
     def __init__(self, name=__name__, level=logging.DEBUG):
         """Creates a logger with a name and loggging level."""
-        log_handler = logging.StreamHandler()
-        log_handler.setLevel(logging.DEBUG)
-        log_handler.setFormatter(logging.Formatter('%(message)s'))
 
         self.logger = logging.getLogger(name)
-        self.logger.addHandler(log_handler)
         self.logger.setLevel(level)
+
         # Avoid duplicate message in AWS cloudwatch
         self.logger.propagate = False
+        if not self.logger.handlers:
+            log_handler = logging.StreamHandler()
+            log_handler.setLevel(logging.DEBUG)
+            log_handler.setFormatter(logging.Formatter('%(message)s'))
+            self.logger.addHandler(log_handler)
+
         self.event = None
         self.context = None
         self._msg = {}