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/unit/config/config_test.py

@@ -775,6 +775,24 @@ class ConfigTest(unittest.TestCase):
             'extends': {'service': 'foo'}
         }
 
+    def test_external_volume_config(self):
+        config_details = build_config_details({
+            'version': 2,
+            'services': {
+                'bogus': {'image': 'busybox'}
+            },
+            'volumes': {
+                'ext': {'external': True},
+                'ext2': {'external': {'name': 'aliased'}}
+            }
+        })
+        config_result = config.load(config_details)
+        volumes = config_result.volumes
+        assert 'ext' in volumes
+        assert volumes['ext']['external'] is True
+        assert 'ext2' in volumes
+        assert volumes['ext2']['external']['name'] == 'aliased'
+
 
 class PortsTest(unittest.TestCase):
     INVALID_PORTS_TYPES = [