Use ebtables-legacy on Ubuntu Focal

On Ubuntu 20.04 there is by default ebtables-nft used. The problem with
that is that nftables don't supports syntax for source and destination
ipv4 address in arp tables right now.
So to workaround/fix this problem at least in our gate jobs we have to
switch to ebtables-legacy for now.
And that's exactly what this patch is doing.

Change-Id: I0aca119f1feca1d8f31a0104dde40d5d52c1b25c
Closes-Bug: #1889779

playbooks/configure_ebtables.yaml

@@ -0,0 +1,5 @@
+- hosts: all
+  roles:
+    # TODO(slaweq): remove it when nftables will support syntax for src and
+    # destination IP addresses in arp tables:
+    - legacy_ebtables

playbooks/configure_functional_job.yaml

@@ -1,5 +1,8 @@
 - hosts: all
   roles:
     - ensure-tox
+    # TODO(slaweq): remove it when nftables will support syntax for src and
+    # destination IP addresses in arp tables:
+    - legacy_ebtables
     - setup_logdir
     - configure_functional_tests

roles/legacy_ebtables/tasks/main.yaml

@@ -0,0 +1,16 @@
+- name: Ensure legacy ebtables is installed
+  package:
+    name: 'ebtables'
+    state: latest
+  become: yes
+
+- name: Switch to legacy ebtables
+  shell:
+    cmd: /usr/bin/update-alternatives --set ebtables /usr/sbin/ebtables-legacy
+    executable: /bin/bash
+  become: yes
+  when:
+    - ansible_facts['distribution_release'] == "focal"
+    - ansible_facts['distribution'] == "Ubuntu"
+
+

zuul.d/tempest-singlenode.yaml

@@ -8,6 +8,7 @@
       - openstack/devstack-gate
       - openstack/neutron
       - openstack/tempest
+    pre-run: playbooks/configure_ebtables.yaml
     vars:
       tempest_concurrency: 4
       devstack_localrc: