Fix multiple handlers causing duplicate messages

[michaeljb]

• test_logger setup: allow kwargs to be forwarded to CumulusLogger constructor
• add test asserting each CumulusLogger instance for a given name should only have one handler
• CumulusLogger constructor: only create new handler if one is not present
  • code style: whitespace added and lines rearranged so that the 'Avoid duplicate message' comment makes the most sense

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 to see duplicates in
CloudWatch. The dmrpp-generator code isn't doing anything out of the ordinary when
creating its CumulusLogger instance, which is what lead me to search here to find the
cause of the duplication.

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"}