Add veth-pair role

This role configures a veth pair between an OVS bridge and another
device.

ansible/roles/veth-pair/README.md

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+Veth Pair
+=========
+
+This role creates a veth pair. It will plug one end into the specified OVS
+bridge and, optionally, can plug the other end into a source Linux bridge.
+
+Requirements
+------------
+
+The host should have the `ip` and `ovs-vsctl` commands accessible. If
+`veth_pair_plug_into_source` is enabled, the command `brctl` must also be
+accessible.
+
+Role Variables
+--------------
+
+- `veth_pair_ovs_link_name`: The name to give the veth link that plugs into the
+  OVS bridge.
+- `veth_pair_ovs_bridge`: The name of the OVS bridge to plug into.
+- `veth_pair_source_link_name`: The name to give the veth link that plugs into
+  the source device.
+- `veth_pair_source_bridge`: The name of the source Linux bridge to plug into. Must be
+  specified if and only if `plug_into_source` is enabled.
+- `plug_into_source`: Whether or not to plug the source end of the veth pair
+  into a Linux bridge. If enabled, `source_bridge`, `source_bridge` must also
+  be specified. Default is `false`.

ansible/roles/veth-pair/defaults/main.yml

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+---
+# Whether or not to plug the source end of the veth pair into a Linux bridge.
+veth_pair_plug_into_source: false

ansible/roles/veth-pair/tasks/main.yml

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+---
+- name: Create veth pair
+  command: >
+    ip link add dev {{ veth_pair_ovs_link_name }}
+    type veth
+    peer name {{ veth_pair_source_link_name }}
+  register: res
+  changed_when: res.rc == 0
+  # Return code 2 means the veth pair already exists
+  failed_when: res.rc not in [0, 2]
+  become: true
+
+- name: Plug veth into OVS bridge
+  openvswitch_port:
+    bridge: "{{ veth_pair_ovs_bridge }}"
+    port: "{{ veth_pair_ovs_link_name }}"
+  become: true
+
+- name: Plug veth into source bridge
+  command: >
+    brctl addif {{ veth_pair_source_bridge }}
+    {{ veth_pair_source_link_name }}
+  register: res
+  failed_when:
+    - res.rc != 0
+    - "'already a member of a bridge' not in res.stderr"
+  changed_when: "'already a member of a bridge' not in res.stderr"
+  when: veth_pair_plug_into_source | bool
+  become: true