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
Merge "Create a directory for servicegroup drivers."