VXLAN is necessary to run HA in CI (due to floating VIP
address handled by keepalived).
It also turned out to be required to have private
IPv6 address assignments.
This patch is based on linux bridge rather than OVS
to avoid problems with OVS deployed in containers.

This patch enables haproxy in multinode jobs.

Includes saving of linux networking details.

Makes DASHBOARD_URL agree with OS_AUTH_URL - properly uses the
pre-upgrade value for testing.

Co-authored-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Depends-on: https://review.opendev.org/683068
Depends-on: https://review.opendev.org/682957
Change-Id: I66888712da80c3d6f84ee4949762961664d3adea