Support Extra Specs to Subscription-confirming

This requirement came from a true scenario when
people use the subscription function of Zaqar,
for example, text messages, they need to return
some extra information like message authentication
code in confirming process.

Change-Id: Ia2a665207d4b267798d9d998d0284c2e68b247a4
Implements: blueprint support-extra-specs-to-subscription-confirming

specs/wallaby/support-extra-specs-to-subscription-confirming.rst

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+..
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+
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+
+==============================================
+Support Extra Specs to Subscription-confirming
+==============================================
+
+https://blueprints.launchpad.net/zaqar/+spec/support-extra-specs-to-subscription-confirming
+
+This requirement came from a true scenariowhen people use the subscription
+function of Zaqar, for example, text messages, they need to return some extra
+information like message authentication code in confirming process.
+Now Zaqar still cant support it, that will impact the usage of subscription.
+
+Problem description
+===================
+
+Currently, Zaqar can't support extra information in subscription
+confirming process. This will block user to input information that
+is needed when subcription is confirming. There is a true case that
+came from Zaqar's user. They want to input the message authentication
+code into Zaqar when the subscription is confirming, the code will be
+used to identify the subscriber. So Zaqar should support this kind of
+mechanism.
+
+Proposed change
+===============
+
+1. Introduce a key-value called "extra_spec" in confirming request body.
+
+2. Introduce a driver mechanism to let vendors to implement what they
+   want to do with extra_spec information.
+
+API Impact
+-----------
+
+Subscription confirming request:
+
+.. code-block::
+
+  PUT: /v2/subscriptions/subscription_id/confirm
+
+    RESPONSE CODE: 204
+    REQUEST BODY:
+    {
+      "confirmed": true,
+      "extra_spec": {"message_authentication_code": "xxxxxx"}
+    }
+
+Drawbacks
+---------
+
+None
+
+Alternatives
+------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  wanghao <sxmatch1986@gmail.com>
+
+Secondary assignee:
+  None
+
+Milestones
+----------
+
+Target Milestone for completion:
+  wallaby M-2
+
+Work Items
+----------
+
+#. Modify api and transport code.
+#. Add driver mechanism to handler the extra spec.
+#. Add release note for this feature.
+#. Update API reference.
+#. Change unit, functional and tempest tests accordingly.
+#. Add client support.
+
+Dependencies
+============
+
+None
+
+References
+==========
+
+None