Deprecate unlimited compute actions

We already deprecated the unlimited max_concurrent_live_migrations
config value and now we do the same for max_concurrent_builds and
max_concurrent_snapshots as well. The reason is similar.
* The unlimited meaning was a lie, it was limited by other constructs in
  the code. For these option the limit was the size of the RPC executor
  defaulted to 64.
* In native threading mode having unlimited concurrent tasks is
  unfeasible due to the memory cost of native threads for each task.

The deprecation is done in a way that in eventlet mode we keep a similar
behavior as before but in native threading mode we enforce a strict
maximum even if unlimited is requested.

Change-Id: Ibbf76c2c85729820035c9791719bf2c864bce12b
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

releasenotes/notes/deprecate-unlimited-max_concurrent_live_migrations-29c54c7eeb77041c.yaml

@@ -18,6 +18,24 @@ upgrade:
     higher value explicitly before the upgrade. Please read the
     `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
     guide for more details.
+  - |
+    The meaning of the 0 value of the config option
+    ``[DEFAULT]max_concurrent_builds`` has been changed. In the past
+    it meant "unlimited" but actually it was limited by the oslo.messaging's
+    ``executor_thread_pool_size`` config option. For eventlet mode "unlimited"
+    now means 1000 concurrent builds. For the native threading mode it is
+    now reduced to 10 native threads. Please also read the `concurrency
+    <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details.
+  - |
+    The meaning of the 0 value of the config option
+    ``[DEFAULT]max_concurrent_snapshots`` has been changed. In the past
+    it meant "unlimited" but actually it was limited by the oslo.messaging's
+    ``executor_thread_pool_size`` config option. For eventlet mode "unlimited"
+    now means 1000 concurrent snapshots. For the native threading mode it is
+    now reduced to 5 native threads. Please also read the `concurrency
+    <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details.
 deprecations:
   - |
     The possible 0 value of the configuration option
@@ -26,3 +44,11 @@ deprecations:
     default value, 1, of this config option. If more performant live migration
     is needed, use the ``live_migration_parallel_connections`` config option
     instead.
+  - |
+    The possible 0 value of the configuration option
+    ``[DEFAULT]max_concurrent_builds`` is deprecated and will be
+    removed in a future release.
+  - |
+    The possible 0 value of the configuration option
+    ``[DEFAULT]max_concurrent_snapshots`` is deprecated and will be
+    removed in a future release.