Initialize root logger in _setup_logging_from_conf()

log.setup() doesn't really initialize root logger, therefore logging
in 3rd party library such as stevedore doesn't work properly. This
patch enable root logger initialization.

Fix bug: # 1073241

Change-Id: I6fa41b2ba41ff97ad0280f8774a26887d294423e
Merge "Initialize root logger in _setup_logging_from_conf()"