Add Placement helper

This patch added Placement to Watcher

Change-Id: I5d200978f2e2c2aa04148109b9252df696bf2132
Implements: blueprint support-placement-api

specs/train/approved/support-placement-api.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=====================
+Support Placement API
+=====================
+
+https://blueprints.launchpad.net/watcher/+spec/support-placement-api
+
+
+Problem description
+===================
+
+Placement provides a service for managing, selecting, and claiming
+available resources in a cloud. It was introduced in the Newton release
+within the nova repository and extracted to the placement repository
+in the Stein release.
+Now Watcher gets data directly from Nova for building data model
+and strategies processing. But some data, such as overcommit allocation
+ratio for VCPU and Ram, can't get from Nova. The knowledge available
+to strategies to make informed decisions is currently limited due to
+the placement api being unavailable.
+
+Use Cases
+----------
+
+As a Watcher developer, I want to get data from Placement.
+
+
+Proposed change
+===============
+
+In this spec we just add Placement helper to Watcher.
+We plan to improve the data model and strategies in
+the future specs.
+The functions in the helper as below:
+
+* List resource providers
+* List resource classes
+* List resource provider inventories
+* List resource provider traits
+* List resource provider allocations
+* List resource provider usages
+* List allocation candidates
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+Add a new config section [placement_client].
+
+Some config options:
+
+* api_version: The minimum version restricted to a given Major API.
+* interface: The default interface for URL discovery.
+* region_name: The default region_name for URL discovery.
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  licanwei
+
+Work Items
+----------
+
+* Add Placement helper
+* Add unit test
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Unittest for Placement helper
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+https://developer.openstack.org/api-ref/placement/
+
+
+History
+=======
+
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Train
+     - Introduced
+