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>

compose/volume.py

@@ -5,12 +5,19 @@ from docker.errors import NotFound
 
 
 class Volume(object):
-    def __init__(self, client, project, name, driver=None, driver_opts=None):
+    def __init__(self, client, project, name, driver=None, driver_opts=None,
+                 external=False):
         self.client = client
         self.project = project
         self.name = name
         self.driver = driver
         self.driver_opts = driver_opts
+        self.external_name = None
+        if external:
+            if isinstance(external, dict):
+                self.external_name = external.get('name')
+            else:
+                self.external_name = self.name
 
     def create(self):
         return self.client.create_volume(
@@ -23,15 +30,19 @@ class Volume(object):
     def inspect(self):
         return self.client.inspect_volume(self.full_name)
 
-    @property
-    def is_user_created(self):
+    def exists(self):
         try:
-            self.client.inspect_volume(self.name)
+            self.inspect()
         except NotFound:
             return False
-
         return True
 
+    @property
+    def external(self):
+        return bool(self.external_name)
+
     @property
     def full_name(self):
+        if self.external_name:
+            return self.external_name
         return '{0}_{1}'.format(self.project, self.name)