Scan for mac through all devices

The first device the agent happens to pick could be something
without a mac address like a 6in6 interface. This was causing
the agent to fail to start if it was unlucky enough to pick
that address.

This patch just adjusts the logic to keep iterating through
the list until we find a mac.

Closes-Bug: #1669087
Change-Id: I6e934a2dff8fd441e1c70c9a5857fd6150835e2a

neutron/plugins/ml2/drivers/linuxbridge/agent/linuxbridge_neutron_agent.py

@@ -716,8 +716,10 @@ class LinuxBridgeManager(amb.CommonAgentManagerBase):
                 list(self.bridge_mappings.values())[0])
         else:
             devices = ip_lib.IPWrapper().get_devices(True)
-            if devices:
-                mac = ip_lib.get_device_mac(devices[0].name)
+            for device in devices:
+                mac = ip_lib.get_device_mac(device.name)
+                if mac:
+                    break
             else:
                 LOG.error(_LE("Unable to obtain MAC address for unique ID. "
                               "Agent terminated!"))

neutron/tests/unit/plugins/ml2/drivers/linuxbridge/agent/test_linuxbridge_neutron_agent.py

@@ -871,11 +871,11 @@ class TestLinuxBridgeManager(base.BaseTestCase):
                 mock.patch.object(
                     ip_lib,
                     "get_device_mac",
-                    return_value='16:63:69:10:a0:59') as mock_gim:
+                    side_effect=[None, '16:63:69:10:a0:59']) as mock_gim:
 
             agent_id = lbm.get_agent_id()
             self.assertEqual("lb16636910a059", agent_id)
-            mock_gim.assert_called_with("eth1")
+            mock_gim.assert_has_calls([mock.call("eth1"), mock.call("eth2")])
 
 
 class TestLinuxBridgeRpcCallbacks(base.BaseTestCase):