Fix veth-pair role for teardown

* Don't delete the OVS bridge specified, as it will already be deleted
  when the role is called.
* Clarify the behaviour of the role for each value of `cmd` in the
  README.
* Succintify main.yml.

ansible/roles/veth-pair/README.md

@@ -1,12 +1,16 @@
 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. If
-`veth_pair_state` is `absent`, it will ensure the veth pair is not plugged into
-the OVS bridge; if `veth_pair_plug_into_source` is enabled, it will ensure the
-veth pair is not plugged into the source bridge; finally, it will ensure the
-veth pair itself does not exist.
+This role manages a veth pair. Actions:
+
+  * If `veth_pair_state` is `present`, it will create the veth pair and
+    plug one end into the specified OVS bridge. If `veth_pair_plug_into_source`
+    is enabled, it will also plug the other end into/from a source Linux
+    bridge.
+
+  * If `veth_pair_state` is `absent`, it will ensure the veth pair does not exist; if
+    `veth_pair_plug_into_source` is also enabled, it will ensure the veth pair is
+    not plugged into the source bridge.
 
 Requirements
 ------------

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

@@ -1,11 +1,4 @@
 ---
-- name: Unplug veth from OVS bridge
-  openvswitch_port:
-    bridge: "{{ veth_pair_ovs_bridge }}"
-    port: "{{ veth_pair_ovs_link_name }}"
-    state: absent
-  become: true
-
 - name: Unplug veth from source bridge
   command: >-
     brctl delif {{ veth_pair_source_bridge }}

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

@@ -1,8 +1,2 @@
 ---
-- name: Ensure veth pair is absent
-  include_tasks: absent.yml
-  when: veth_pair_state == 'absent'
-
-- name: Ensure veth pair is present
-  include_tasks: present.yml
-  when: veth_pair_state != 'absent'
+- include_tasks: "{{ veth_pair_state }}.yml"