Fix volume merging when there's no explicit host path

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

compose/config.py

@@ -324,15 +324,24 @@ def dict_from_volumes(volumes):
         return {}
 
 
-def split_volume(volume):
-    if ':' in volume:
-        return reversed(volume.split(':', 1))
-    else:
-        return (volume, None)
-
-
 def volumes_from_dict(d):
-    return ["%s:%s" % (host_path, container_path) for (container_path, host_path) in d.items()]
+    return [join_volume(v) for v in d.items()]
+
+
+def split_volume(string):
+    if ':' in string:
+        (host, container) = string.split(':', 1)
+        return (container, host)
+    else:
+        return (string, None)
+
+
+def join_volume(pair):
+    (container, host) = pair
+    if host is None:
+        return container
+    else:
+        return ":".join((host, container))
 
 
 def expand_path(working_dir, path):