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/master_jobs.yaml b/zuul.d/master_jobs.yaml
index 45862c8..ab6973c 100644
--- a/zuul.d/master_jobs.yaml
+++ b/zuul.d/master_jobs.yaml
@@ -240,6 +240,7 @@
           $TEMPEST_CONFIG:
             neutron_plugin_options:
               available_type_drivers: local,flat,vlan,geneve
+              ipv6_metadata: False
               is_igmp_snooping_enabled: True
 
 - job: