Do a better job of quietening the logs

We were overriding the "defaut_log_levels" option and losing
some useful options. This just extends the list instead of
overwriting it.

Also added:
- stevedore
- periodic debugs

Change-Id: Iacef411f896854751beb44488ed4b2a8a7268fa8
Merge "Do a better job of quietening the logs"