Fix bugs with one-off legacy containers

- One-off containers were included in the warning log messages, which can
  make for unreadable output when there are lots (as there often are).

- Compose was attempting to recreate one-off containers as normal
  containers when migrating.

Fixed by implementing the exact naming logic from before we used labels.

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

tests/integration/legacy_test.py

@@ -17,6 +17,7 @@ class ProjectTest(DockerClientTestCase):
 
         self.project = Project('composetest', self.services, self.client)
 
+        # Create a legacy container for each service
         for service in self.services:
             service.ensure_image_exists()
             self.client.create_container(
@@ -24,6 +25,12 @@ class ProjectTest(DockerClientTestCase):
                 **service.options
             )
 
+        # Create a single one-off legacy container
+        self.client.create_container(
+            name='{}_{}_run_1'.format(self.project.name, self.services[0].name),
+            **self.services[0].options
+        )
+
     def get_names(self, **kwargs):
         if 'stopped' not in kwargs:
             kwargs['stopped'] = True
@@ -38,6 +45,9 @@ class ProjectTest(DockerClientTestCase):
     def test_get_legacy_container_names(self):
         self.assertEqual(len(self.get_names()), len(self.services))
 
+    def test_get_legacy_container_names_one_off(self):
+        self.assertEqual(len(self.get_names(one_off=True)), 1)
+
     def test_migration_to_labels(self):
         with mock.patch.object(legacy, 'log', autospec=True) as mock_log:
             self.assertEqual(self.project.containers(stopped=True), [])