Disable flavor_instance_plugin testing by default

We have CI issues on stable branches where we try to test flavor-based
instance reservation even though it is not supported in these releases.
We can enable this testing by default once we drop support for releases
older than 2024.2 (Dalmatian).

Change-Id: Icc97572f0d30eff6ab77e667952a83f7f683ef84

.zuul.yaml

@@ -79,6 +79,12 @@
     parent: blazar-tempest-plugin-base
     nodeset: openstack-single-node-jammy
     override-checkout: stable/2024.2
+    vars:
+      devstack_local_conf:
+        test-config:
+          $TEMPEST_CONFIG:
+            resource_reservation:
+              flavor_instance_plugin: true
 
 - job:
     name: blazar-tempest-plugin-2024-1

blazar_tempest_plugin/config.py

@@ -37,7 +37,7 @@ ResourceReservationGroup = [
                help="The endpoint type to use for the resource_reservation "
                     "service."),
     cfg.BoolOpt('flavor_instance_plugin',
-                default=True,
+                default=False,
                 help="Whether to test flavor-based instance reservation"),
     cfg.IntOpt('lease_interval',
                default=10,