Add volume type filter to Get-Pools

Add feature that administrators can get back-end storage pools
filtered by volume-type, underground cinder will return the pools
filtered by volume-type's extra-spec.
This is depended on generalized  resource filtering feature [1].

[1] https://review.openstack.org/#/c/441516/

Change-Id: I8ed01bbee0c1f54f1761c812987ac70b4c4bbbcf
blueprint: add-volume-type-filter-to-get-pool

specs/pike/add-volume-type-filter-to-get-pools.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 retrieve pools filtered by volume-type
+==============================================
+
+https://blueprints.launchpad.net/cinder/+spec/add-volume-type-filter-to-
+get-pool
+
+Add feature that administrators can get back-end storage pools filtered by
+volume-type, cinder will return the pools filtered by volume-type's
+extra-specs.
+
+Problem description
+===================
+
+Now cinder's ``get-pools`` API doesn't support filtering pools by volume type,
+and this is inconvenient when administrators want to know the specified pool's
+state which also can meet volume type's requirements, the administrators also
+can get all pools and filter them on their own, but it's more complicated and
+inefficient. This change intends to cover this situation and bring more
+convenience to administrators.
+
+Use Cases
+=========
+
+In production environment, administrators often need to have an overall
+pool available statistics filtered by volume type, this will help them to make
+an adjustment before resources run out.
+
+Proposed change
+===============
+
+As we will introduce generalized resource filter in cinder, from the view of
+outside cinder, the only thing we should do is to advertise that we can
+support this filter now:
+
+From the view of inside cinder. We will mark ``volume-type`` recognizable, and
+support for this filter in logic:
+
+* once the volume-type (name or id) is detected, the volume type object will be
+  retrieved before scheduler api is called, and will be passed as a filter
+  item.
+
+* the ``scheduler.get_pools`` called, which will call the
+  ``host_manager.get_pools`` in result.
+
+* the ``host_manager.get_pools`` will collect the pools information as normal,
+  and before it returns, the result will be filtered by
+  ``host.get_filtered_hosts``.
+
+* the filter properties of ``get_filtered_hosts`` only consists of volume-type
+  properties.
+
+* As already proposed by generalized resource filtering `[1]`, the changes on
+  cinder-client for this feature are not needed.
+
+
+Alternatives
+------------
+
+Administrators also can retrieve and filter on their own, but it's more
+complicated and inefficient. This change can reduce the request amount and
+filter unnecessary data transmitted from server to client.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+Get-Pool API will accept new query string parameter volume-type.
+Administrators can pass name or ID to retrieve pools filtered.
+
+* ``GET /v3/{tenant_id}/scheduler-stats/get_pools?volume-type=lvm-default``
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None.
+
+Other end user impact
+---------------------
+
+Within generalized resource filtering, we would ultimately have a
+``get-pools`` command like this below::
+
+ cinder get-pools --filters volume-type='volume type'
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  TommyLike(tommylikehu@gmail.com)
+
+Work Items
+----------
+
+* Add Get-Pools's filter.
+* Add filter logic when retrieving pools.
+* Add related tests.
+
+
+Dependencies
+============
+
+Depended on generalized resource filtering `[1]`_
+
+Testing
+=======
+
+1. Unit test to test whether volume-type filter can be correctly applied.
+2. Tempest test whether volume-type filter work correctly from API
+   perspective.
+
+Documentation Impact
+====================
+
+1. The cinder API documentation will need to be updated to reflect the
+   REST API changes.
+
+References
+==========
+
+_`[1]`: https://review.openstack.org/#/c/441516/