Yagna datetime format in logs

[azawlocki]

Resolves #388

Example log line after the changes in this PR:

[2021-06-11T14:54:54.734.995+0200 DEBUG yapapi] Yapapi version: 0.6.0-alpha.1, script: examples/three_clusters.py, working directory: /home/azawlocki/golem/yapapi

[filipgolem]

There is no need to display microseconds.

Suggested change

	"""Format datetime; example: `2021-06-11T14:55:43.156.123+0200`."""
	dt = datetime.fromtimestamp(record.created, tz=self.LOCAL_TZ)
	return dt.strftime(f"%Y-%m-%dT%H:%M:%S.{dt.microsecond / 1000}%z")
	"""Format datetime; example: `2021-06-11T14:55:43.156+0200`."""
	dt = datetime.fromtimestamp(record.created, tz=self.LOCAL_TZ)
	return dt.strftime(f"%Y-%m-%dT%H:%M:%S.{int(dt.microsecond / 1000)}%z")

[azawlocki]

Thanks, @filipgolem, good catch! And I need also to remember about the leading 0's, in case the number of milliseconds is less than 100.