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
(cherry picked from commit 47f9fb16cb)

zun/objects/container.py

@@ -418,6 +418,11 @@ 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()