Ability to customize default_log_levels for each project

Add a new parameter default_log_levels to log.set_defaults. We
need to default its value to None since this is a backwards
incompatible change otherwise. However, if someone is not setting
a specific value, we do need to set the usual default levels

exposed DEFAULT_LOG_LEVELS public variable so others can use
the default log levels to construct their custom defaults.

Change-Id: I49a82c5b3446784d254ca558f51b6c3e5c2028eb
Closes-Bug: #1297950
Merge "Ability to customize default_log_levels for each project"