Fix 'unable to exclude instance'

Change-Id: I1599a86a2ba7d3af755fb1412a5e38516c736957
Closes-Bug: #1736129

watcher/decision_engine/scope/compute.py

@@ -147,12 +147,16 @@ class ComputeScope(base.BaseScope):
         nodes_to_remove = set()
         instances_to_exclude = []
         instance_metadata = []
+        compute_scope = []
         model_hosts = list(cluster_model.get_all_compute_nodes().keys())
 
-        if not self.scope:
+        for scope in self.scope:
+            compute_scope = scope.get('compute')
+
+        if not compute_scope:
             return cluster_model
 
-        for rule in self.scope:
+        for rule in compute_scope:
             if 'host_aggregates' in rule:
                 self._collect_aggregates(rule['host_aggregates'],
                                          allowed_nodes)

watcher/tests/decision_engine/scope/fake_scopes.py

@@ -14,10 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-fake_scope_1 = [{'availability_zones': [{'name': 'AZ1'}]},
-                {'exclude':
-                    [{'instances':
-                        [{'uuid': 'INSTANCE_6'}]}]
+fake_scope_1 = [{'compute': [{'availability_zones': [{'name': 'AZ1'}]},
+                             {'exclude': [
+                                 {'instances': [
+                                     {'uuid': 'INSTANCE_6'}]},
+                             ]}]
                  }
                 ]