Sort instances if possible inside an host aggregate

Simple use case: Reduce the licence cost
Windows licences are billed by host or by socket. The goal is to stack all
Windows instances on the same hosts as much as possible without creating a
new host aggregate.

As much as possible means this is a best effort feature, if nova can place
this new Windows instance on a compute where other Windows are running, do
it. If not, spawn it where you want or on a empty compute to identify it
as « preferred widows compute » for future windows spawn.

This use case could be extended to many other situations, every time you
have a preferred situation with no hard restrictions.

Co-Authored-By: Michael Still <mikal@stillhq.com>

Change-Id: I261bcb802573406f0aca5a87606e4dcca877301b

specs/backlog/approved/same-instances-scheduling.rst

@@ -0,0 +1,139 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===================================================
+Sort instances if possible inside an host aggregate
+===================================================
+
+https://blueprints.launchpad.net/nova/+spec/same-instances-scheduling
+
+Nova should allow sorting instances based on preferences without
+host aggregate speraration.
+
+Problem description
+===================
+
+As an operator, when you manage hundred or thousand nodes, instances will be
+automaticly balanced though all nodes. There is no possibility to ask the
+infrastructure to group a kind of instances on the same hosts if possible.
+
+A kind of instance could be based on a specific image, a specific flavor.
+
+For this use case, host-aggregates are not used because this is not a strong
+constraint, this is a prefered situation.
+
+Use Cases
+----------
+
+Reduce the licence cost: Microsoft Windows licences are billed by host or by
+socket. The goal is to stack all Windows instances on the same hosts as much as
+possible.
+
+As much as possible means this is a best effort feature, if Nova can place
+this new Windows instance on a compute where other Windows are running, do
+it. If not, spawn it where you want or on a empty compute to identify it
+as `preferred widows compute` for future windows spawn.
+
+Project Priority
+-----------------
+
+None
+
+Proposed change
+===============
+
+Posting to get preliminary feedback on the scope of this spec.
+
+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
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+None
+
+Work Items
+----------
+
+None
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+None
+
+Documentation Impact
+====================
+
+None
+
+References
+==========
+
+None
+
+History
+=======
+
+None
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Liberty
+     - Introduced