Test metadata over IPv6

To ssh into the instance we use an IPv4 floating IP,
but then we test accessing metadata over IPv6.

tempest.conf:

[neutron_plugin_options]
advanced_image_ref = ubuntu 16.04 does not but 18.04 and 20.04 work
advanced_image_flavor_ref = d1 # ds512M
advanced_image_ssh_user = ubuntu
default_image_is_advanced = False

Introduce new tempest(-plugin) boolean config option:
    neutron_plugin_options.ipv6_metadata
    Defaults to True.
This controls whether the test should run since API extension based
auto-discovery is naturally unavailable for the feature.

Disable executing the test in ovn and older-than-victoria gate jobs.

Change-Id: I7a1d2057489067e555cb247e473ad11e63dbdd86
Related-Bug: #1460177
diff --git a/zuul.d/stein_jobs.yaml b/zuul.d/stein_jobs.yaml
index 1c9e299..745b40f 100644
--- a/zuul.d/stein_jobs.yaml
+++ b/zuul.d/stein_jobs.yaml
@@ -84,6 +84,11 @@
       network_api_extensions: *api_extensions
       devstack_localrc:
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
+      devstack_local_conf:
+        test-config:
+          $TEMPEST_CONFIG:
+            neutron_plugin_options:
+              ipv6_metadata: False
 
 - job:
     name: neutron-tempest-plugin-scenario-openvswitch-iptables_hybrid-stein
@@ -94,6 +99,11 @@
       network_api_extensions: *api_extensions
       devstack_localrc:
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
+      devstack_local_conf:
+        test-config:
+          $TEMPEST_CONFIG:
+            neutron_plugin_options:
+              ipv6_metadata: False
 
 - job:
     name: neutron-tempest-plugin-scenario-linuxbridge-stein
@@ -104,6 +114,11 @@
       network_api_extensions: *api_extensions
       devstack_localrc:
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
+      devstack_local_conf:
+        test-config:
+          $TEMPEST_CONFIG:
+            neutron_plugin_options:
+              ipv6_metadata: False
 
 - job:
     name: neutron-tempest-plugin-dvr-multinode-scenario-stein