Add tests for some db.security_group_* methods
Add tests for some db.security_group_* methods

Introduce SecurityGroupTestCase with tests for
some of the db.security_group_* methods, such as
create, destroy, get, etc.

blueprint db-api-tests

Change-Id: I596c85e3d117fa255e457e05aa845cd1118a2e86

n0ano/gantt,n0ano/gantt