make 0 be avaiable value of options

value 0 is regard as not available by swift-ring-builder
$ swift-ring-builder testring add --region 0 --zone 1
--ip 127.0.0.2 --port 6000 --device sdb --weight 100

Required argument -r/--region not specified.
The on-disk ring builder is unchanged.

this patch is to make value 0 available.

Change-Id: Id941d44d8dbfe438bf921ed905908b838c88a644
Closes-bug: #1547137

swift/common/ring/utils.py

@@ -616,7 +616,7 @@ def build_dev_from_opts(opts):
                                          ['port', '-p', '--port'],
                                          ['device', '-d', '--device'],
                                          ['weight', '-w', '--weight']):
-        if not getattr(opts, attribute, None):
+        if getattr(opts, attribute, None) is None:
             raise ValueError('Required argument %s/%s not specified.' %
                              (shortopt, longopt))
 

test/unit/common/ring/test_utils.py

@@ -579,7 +579,7 @@ class TestUtils(unittest.TestCase):
 
     def test_build_dev_from_opts(self):
         argv = \
-            ["--region", "2", "--zone", "3",
+            ["--region", "0", "--zone", "3",
              "--ip", "test.test.com",
              "--port", "6200",
              "--replication-ip", "r.test.com",
@@ -588,7 +588,7 @@ class TestUtils(unittest.TestCase):
              "--meta", "some meta data",
              "--weight", "3.14159265359"]
         expected = {
-            'region': 2,
+            'region': 0,
             'zone': 3,
             'ip': "test.test.com",
             'port': 6200,