Implement ability to specify external volumes

External volumes are created and managed by the user.
They are not namespaced.
They are expected to exist at the beginning of the up phase.

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/integration/volume_test.py

@@ -18,9 +18,10 @@ class VolumeTest(DockerClientTestCase):
             except DockerException:
                 pass
 
-    def create_volume(self, name, driver=None, opts=None):
+    def create_volume(self, name, driver=None, opts=None, external=False):
         vol = Volume(
-            self.client, 'composetest', name, driver=driver, driver_opts=opts
+            self.client, 'composetest', name, driver=driver, driver_opts=opts,
+            external=external
         )
         self.tmp_volumes.append(vol)
         return vol
@@ -55,12 +56,19 @@ class VolumeTest(DockerClientTestCase):
         volumes = self.client.volumes()['Volumes']
         assert len([v for v in volumes if v['Name'] == vol.full_name]) == 0
 
-    def test_is_user_created(self):
-        vol = Volume(self.client, 'composetest', 'uservolume01')
-        try:
-            self.client.create_volume('uservolume01')
-            assert vol.is_user_created is True
-        finally:
-            self.client.remove_volume('uservolume01')
-        vol2 = Volume(self.client, 'composetest', 'volume01')
-        assert vol2.is_user_created is False
+    def test_external_volume(self):
+        vol = self.create_volume('volume01', external=True)
+        assert vol.external is True
+        assert vol.full_name == vol.name
+        vol.create()
+        info = vol.inspect()
+        assert info['Name'] == vol.name
+
+    def test_external_aliased_volume(self):
+        alias_name = 'alias01'
+        vol = self.create_volume('volume01', external={'name': alias_name})
+        assert vol.external is True
+        assert vol.full_name == alias_name
+        vol.create()
+        info = vol.inspect()
+        assert info['Name'] == alias_name