Deprecate nova api extensions config option

Now that the nova extensions mechanism is deprecated and api extensions
aren't really a thing anymore we should deprecate the corresponding
mechanism in tempest. Moving forward there really isn't anything to set
here because microversions are the api selection mechanism and the
"extension list" is fixed. We can remove this option when no supported
nova release contains extensions anymore.

Change-Id: I1286294b1b61aebf53437b37f6c141bc4c587b1f
diff --git a/releasenotes/notes/deprecate-nova-api-extensions-df16b02485dae203.yaml b/releasenotes/notes/deprecate-nova-api-extensions-df16b02485dae203.yaml
new file mode 100644
index 0000000..c2d9a9b
--- /dev/null
+++ b/releasenotes/notes/deprecate-nova-api-extensions-df16b02485dae203.yaml
@@ -0,0 +1,7 @@
+---
+deprecations:
+  - The *api_extensions* config option in the *compute-feature-enabled* group is
+    now deprecated. This option will be removed from tempest when all the
+    OpenStack releases supported by tempest no longer support the API extensions
+    mechanism. This was removed from Nova during the Newton cycle, so this will
+    be removed at the Mitaka EOL.