Loadable Cluster Data Model Collectors

In this changeset, I made BaseClusterDataModelCollector instances
pluggable. This corresponds to "part 1" of the work items detailed
in the specifications.

Change-Id: Iab1c7e264add9e2cbbbb767e3fd6e99a0c22c691
Partially-Implements: blueprint cluster-model-objects-wrapper

watcher/decision_engine/model/model_root.py

@@ -13,8 +13,10 @@
 # implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 from watcher._i18n import _
 from watcher.common import exception
+from watcher.common import utils
 from watcher.decision_engine.model import hypervisor
 from watcher.decision_engine.model import mapping
 from watcher.decision_engine.model import vm
@@ -22,10 +24,10 @@ from watcher.decision_engine.model import vm
 
 class ModelRoot(object):
     def __init__(self):
-        self._hypervisors = {}
-        self._vms = {}
+        self._hypervisors = utils.Struct()
+        self._vms = utils.Struct()
         self.mapping = mapping.Mapping(self)
-        self.resource = {}
+        self.resource = utils.Struct()
 
     def assert_hypervisor(self, obj):
         if not isinstance(obj, hypervisor.Hypervisor):