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
9 files changed