available_features for Neutron tempest tests is default to "all" and
used from Victoria to enable/disable tests that has no API extension.
It is added here to make sure that stable gating will not execute these
tests (like for IPv6 metadata in neutron-tempest-plugin or updating
min_bw QoS values for port already attached to VMs)
Cherry-picked from stable/victoria patch
(https://review.opendev.org/c/openstack/devstack/+/769885), but the list
for available_features is empty on Ussuri.
Change-Id: Ic6056218e51e6b0da554bcdf139214d2229ba3c1
Related-Bug: #1882804
(cherry picked from commit e34a8720bf)
ef784e195f