Add compute-nodes-az-sync spec

It proposes an automatic way to sync the Juju availability zones
with the OpenStack availability zones.

Change-Id: I315ffbdb2a1a0ab993fca1c0f39f758b214a36ce

specs/wallaby/approved/compute-nodes-az-sync.rst

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+..
+  Copyright 2021 Canonical Ltd.
+
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode
+
+..
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+
+====================================================
+OpenStack Compute Nodes Juju Availability Zones Sync
+====================================================
+
+For some providers, Juju may set the environment variable
+``JUJU_AVAILABILITY_ZONE`` with information about the availability zone of the
+machine deployed.
+
+OpenStack has the concept of availability zones too, and it would be useful
+for Juju operators to have the option of automatically sync the Juju
+availability zones with the OpenStack availability zones.
+
+Problem Description
+===================
+
+At the moment, a Juju operator needs to manually sync the nova-compute Juju
+machines' availability zones (AZs), with the OpenStack AZs.
+
+Proposed Change
+===============
+
+A new Juju action to the nova-cloud-controller charm will be implemented to
+automatically sync the Juju AZs with the OpenStack AZs. The availability zone
+for each Juju unit is exposed to the nova-cloud-controller charm through the
+cloud-compute relation.
+
+Alternatives
+------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  ionutbalutoiu
+
+Gerrit Topic
+------------
+
+Use Gerrit topic "compute-nodes-az-sync" for all patches related to this spec.
+
+.. code-block:: bash
+
+    git-review -t compute-nodes-az-sync
+
+Work Items
+----------
+
+- nova-compute
+
+  - Set a new relation variable called ``availability_zone`` in the
+    ``cloud-compute`` relation (used to relate to the ``nova-cloud-controller``
+    application). This variable will take the value of the automatic Juju
+    variable ``JUJU_AVAILABILITY_ZONE``, if this is set by the underlying
+    provider.
+
+- nova-cloud-controller
+
+  - Implement a new Juju action called ``sync-compute-availability-zones`` used
+    to sync the Juju availability zones, set by the remote units in the
+    ``cloud-compute`` relation, with the OpenStack availability zones.
+
+Repositories
+------------
+
+- openstack/charm-nova-compute
+
+- openstack/charm-nova-cloud-controller
+
+Documentation
+-------------
+
+The new Juju action needs to be documented in the appropriate docs that
+reference the nova-cloud-controller actions. This is the only user-facing
+change.
+
+Security
+--------
+
+None
+
+Testing
+-------
+
+Code written or changed will be covered by unit tests.
+
+Dependencies
+============
+
+The ``nova-cloud-controller`` charm does not install the Nova client Python
+library. This is needed for the new Juju action, so it is added to the charm
+Python dependencies.