NCBC-394: Add 'Warn' Level Logging to 'core' configuration classes

Change-Id: I117aeb5cf57624e2e012f6a2e8ba0198de6b54d0
Reviewed-on: http://review.couchbase.org/33694
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>