Glance: Allow configuring default_log_levels

Allow a custom value for 'default_log_levels' to be set for both the
Glance registry and API.

Allows setting 'eventlet.wsgi.server=DEBUG' which is useful for tracking
each HTTP request.

Also allows setting a log level for any middleware etc.

Change-Id: I2a791b707fd91ba2d4ca7217b5189969a110c0c9
Merge "Glance: Allow configuring default_log_levels"