Refactor the getid method base.py

Refer to a merged commit.
https://review.openstack.org/#/c/588983/

Change-Id: I37d9f1f0a6b4e0b6493dd12e5599972d1c98f158

magnumclient/common/base.py

@@ -32,10 +32,7 @@ def getid(obj):
     Abstracts the common pattern of allowing both an object or an
     object's ID (UUID) as a parameter when dealing with relationships.
     """
-    try:
-        return obj.id
-    except AttributeError:
-        return obj
+    return getattr(obj, 'id', obj)
 
 
 class Manager(object):