Don't recreate pre-existing volumes.

During the initialize_volumes phase, if a volume using the
non-namespaced name already exists, don't create the namespaced
equivalent.

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

compose/volume.py

@@ -1,6 +1,8 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+from docker.errors import NotFound
+
 
 class Volume(object):
     def __init__(self, client, project, name, driver=None, driver_opts=None):
@@ -21,6 +23,15 @@ class Volume(object):
     def inspect(self):
         return self.client.inspect_volume(self.full_name)
 
+    @property
+    def is_user_created(self):
+        try:
+            self.client.inspect_volume(self.name)
+        except NotFound:
+            return False
+
+        return True
+
     @property
     def full_name(self):
         return '{0}_{1}'.format(self.project, self.name)