start using available_features cfg option from tempest

[1] introduced a new list cfg option to tempest network-feature-enabled
available_features, which can list networking features without and
API extension.
As a first candidate ipv6_metadata can be added to master and victoria
jobs.

[1] https://review.opendev.org/760916

Change-Id: Ib8ba513ad098bb47d401ba64311b92280a85e1b2
diff --git a/zuul.d/rocky_jobs.yaml b/zuul.d/rocky_jobs.yaml
index 6eb8c15..ef794a1 100644
--- a/zuul.d/rocky_jobs.yaml
+++ b/zuul.d/rocky_jobs.yaml
@@ -108,6 +108,8 @@
     vars: &scenario_vars_rocky
       branch_override: stable/rocky
       network_api_extensions: *api_extensions
+      network_available_features: &available_features
+        -
       devstack_localrc:
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"