Create a directory for servicegroup drivers.

Right now there is only one servicegroup driver, the db backed driver.
Create a directory for drivers to reside in to keep things a bit more
tidy as we add additional drivers.

Part of blueprint rpc-based-servicegroup-driver.

Change-Id: Ib563e1a8d184cef838e5730b2fc6904940d04c21

nova/servicegroup/api.py

@@ -39,7 +39,9 @@ CONF.register_opt(servicegroup_driver_opt)
 class API(object):
 
     _driver = None
-    _driver_name_class_mapping = {"db": "nova.servicegroup.db_driver.DbDriver"}
+    _driver_name_class_mapping = {
+        'db': 'nova.servicegroup.drivers.db.DbDriver'
+    }
 
     @lockutils.synchronized('nova.servicegroup.api.new', 'nova-')
     def __new__(cls, *args, **kwargs):