Add the 'containers' field back to capsule

The capsule object is used to have the 'containers' field which
is the list of container objects. This field was accidentally
removed during the merging of data model into container.
This commit adds this field back.

Change-Id: I2cdf365ae4c394eeef6672eed59855f36e7006fc

zun/objects/container.py

@@ -392,6 +392,11 @@ class Capsule(ContainerBase):
                                                      nullable=True),
     }
 
+    def as_dict(self):
+        capsule_dict = super(Capsule, self).as_dict()
+        capsule_dict['containers'] = [c.as_dict() for c in self.containers]
+        return capsule_dict
+
     def obj_load_attr(self, attrname):
         if attrname == 'containers':
             self._load_capsule_containers()