Move auto_assign_nic to class method

Moving logic of auto_assign_nic to class function is to help re-use it
in _boot_servers (it is used in _boot_server only now).

Change-Id: I22e80757b2394fcec63a55da6bee291d3b8fd573
Partial-Bug: #1477583

rally/plugins/openstack/scenarios/nova/utils.py

@@ -96,6 +96,19 @@ class NovaScenario(scenario.OpenStackScenario):
         """Returns user servers list."""
         return self.clients("nova").servers.list(detailed)
 
+    def _pick_random_nic(self):
+        """Choose one network from existing ones."""
+        ctxt = self.context
+        nets = [net["id"]
+                for net in ctxt.get("tenant", {}).get("networks", [])]
+        if nets:
+            # NOTE(amaretskiy): Balance servers among networks:
+            #     divmod(iteration % tenants_num, nets_num)[1]
+            net_idx = divmod(
+                (ctxt["iteration"] % ctxt["config"]["users"]["tenants"]),
+                len(nets))[1]
+            return [{"net-id": nets[net_idx]}]
+
     @atomic.action_timer("nova.boot_server")
     def _boot_server(self, image_id, flavor_id,
                      auto_assign_nic=False, name=None, **kwargs):
@@ -122,16 +135,9 @@ class NovaScenario(scenario.OpenStackScenario):
                 kwargs["security_groups"].append(secgroup["name"])
 
         if auto_assign_nic and not kwargs.get("nics", False):
-            nets = [net["id"] for net in
+            nic = self._pick_random_nic()
-                    self.context.get("tenant", {}).get("networks", [])]
+            if nic:
-            if nets:
+                kwargs["nics"] = nic
-                # NOTE(amaretskiy): Balance servers among networks:
-                #     divmod(iteration % tenants_num, nets_num)[1]
-                net_idx = divmod(
-                    (self.context["iteration"]
-                     % self.context["config"]["users"]["tenants"]),
-                    len(nets))[1]
-                kwargs["nics"] = [{"net-id": nets[net_idx]}]
 
         server = self.clients("nova").servers.create(
             server_name, image_id, flavor_id, **kwargs)

tests/unit/plugins/openstack/scenarios/nova/test_utils.py

@@ -46,6 +46,27 @@ class NovaScenarioTestCase(test.ScenarioTestCase):
         self._test_atomic_action_timer(nova_scenario.atomic_actions(),
                                        "nova.list_servers")
 
+    def test__pick_random_nic(self):
+        context = {"tenant": {"networks": [{"id": "net_id_1"},
+                                           {"id": "net_id_2"}]},
+                   "iteration": 0,
+                   "config": {"users": {"tenants": 2}}}
+        nova_scenario = utils.NovaScenario(context=context)
+        nic1 = nova_scenario._pick_random_nic()
+        self.assertEqual(nic1, [{"net-id": "net_id_1"}])
+
+        context["iteration"] = 1
+        nova_scenario = utils.NovaScenario(context=context)
+        nic2 = nova_scenario._pick_random_nic()
+        # balance to net 2
+        self.assertEqual(nic2, [{"net-id": "net_id_2"}])
+
+        context["iteration"] = 2
+        nova_scenario = utils.NovaScenario(context=context)
+        nic3 = nova_scenario._pick_random_nic()
+        # balance again, get net 1
+        self.assertEqual(nic3, [{"net-id": "net_id_1"}])
+
     @mock.patch(NOVA_UTILS + ".NovaScenario._generate_random_name",
                 return_value="foo_server_name")
     def test__boot_server(self, mock__generate_random_name):