Don't set network mode when none is specified

Setting a value overrides the new default network option.

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

compose/project.py

@@ -188,7 +188,7 @@ class Project(object):
             del service_dict['net']
 
         else:
-            net = 'bridge'
+            net = None
 
         return net