conf: Deprecate '[scheduler] driver'

With the removal of 'CachingScheduler' in Stein (19.0.0), the
'FilterScheduler' is the only in-tree scheduler provided by nova. The
removal of the caching scheduler was motivated in good part because
"more and more functionality within nova [now] relies on managing
(sometimes complex) resource allocations in the placement service". This
same argument applies for out-of-tree drivers. It's unlikely that there
are users with out-of-tree drivers that have good placement integration
and significantly outperform the default filter scheduler. As a result,
it's probably time to close this extension point in favour of continued
improvements on the filter scheduler and placement along with custom
filters and weighters for the filter scheduler.

Change-Id: Id65e0b16fe062509097b8e3f58eb81bfe1cc2bfa
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

nova/conf/scheduler.py

@@ -26,6 +26,15 @@ scheduler_opts = [
         default="filter_scheduler",
         deprecated_name="scheduler_driver",
         deprecated_group="DEFAULT",
+        deprecated_for_removal=True,
+        deprecated_since='21.0.0',
+        deprecated_reason="""
+nova no longer provides any in-tree filters except for the 'filter_scheduler'
+scheduler. This filter is considered flexible and pluggable enough for all use
+cases and can be extended through the use of custom, out-of-tree filters and
+weighers along with powerful, in-tree filters like the
+'AggregateInstanceExtraSpecsFilter' and 'ComputeCapabilitiesFilter' filters.
+""",
         help="""
 The class of the driver used by the scheduler. This should be chosen from one
 of the entrypoints under the namespace 'nova.scheduler.driver' of file

releasenotes/notes/deprecate-scheduler-driver-opt-4d6a266590b52e2c.yaml

@@ -0,0 +1,11 @@
+---
+deprecations:
+  - |
+    The ``[scheduler] driver`` config option has been deprecated. This was
+    previously used to switch between different scheduler drivers including
+    custom, out-of-tree ones. However, only the ``FilterScheduler`` has been
+    supported in-tree since 19.0.0 (Stein) and nova increasingly relies on
+    placement for basic functionality, meaning developing and maintaining
+    out-of-tree drivers is increasingly difficult. Users who still rely on a
+    custom scheduler driver should migrate to the filter scheduler, using
+    custom filters and weighters where necessary.