Allow modules to be registered dynamically

Add a stevedore entrypoint (dragonflow.db.models). Modules containing
models (e.g. the files in dragonflow/db/models) can now be registered
dynamically (i.e. not hard-coded).  The changes to setup.cfg show how
this is done.

Change-Id: I42c0b35e54693b9690771398a99c2d9a4d1c9ab0

dragonflow/db/models/all.py

@@ -9,14 +9,17 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
-from dragonflow.db.models import active_port  # noqa
-from dragonflow.db.models import bgp  # noqa
-from dragonflow.db.models import core  # noqa
-from dragonflow.db.models import l2  # noqa
-from dragonflow.db.models import l3  # noqa
-from dragonflow.db.models import migration  # noqa
-from dragonflow.db.models import qos  # noqa
-from dragonflow.db.models import secgroups  # noqa
-from dragonflow.db.models import service  # noqa
-from dragonflow.db.models import sfc  # noqa
-from dragonflow.db.models import trunk  # noqa
+
+import stevedore
+
+
+def load_all_extensions():
+    """Load all available modules with DF models"""
+    manager = stevedore.ExtensionManager(  # noqa F841
+        'dragonflow.db.models',
+    )
+    # NOTE(oanson) In case Stevedore changes and we need to manually load
+    # the extensions:
+    # modules = [extension.plugin for extension in manager]
+
+load_all_extensions()

setup.cfg

@@ -110,3 +110,15 @@ dragonflow.controller.apps =
     sg = dragonflow.controller.apps.sg:SGApp
     trunk = dragonflow.controller.apps.trunk:TrunkApp
     tunneling = dragonflow.controller.apps.tunneling:TunnelingApp
+dragonflow.db.models =
+    active_port = dragonflow.db.models.active_port
+    bgp = dragonflow.db.models.bgp
+    core = dragonflow.db.models.core
+    l2 = dragonflow.db.models.l2
+    l3 = dragonflow.db.models.l3
+    migration = dragonflow.db.models.migration
+    qos = dragonflow.db.models.qos
+    secgroups = dragonflow.db.models.secgroups
+    service = dragonflow.db.models.service
+    sfc = dragonflow.db.models.sfc
+    trunk = dragonflow.db.models.trunk