api/ec2: make the parameter parser an independent method

Following the review, make the parser of argument items
an independent method for readability.

nova/tests/test_api.py

@@ -89,7 +89,7 @@ class FakeHttplibConnection(object):
 class XmlConversionTestCase(test.TestCase):
     """Unit test api xml conversion"""
     def test_number_conversion(self):
-        conv = apirequest._try_convert
+        conv = ec2utils._try_convert
         self.assertEqual(conv('None'), None)
         self.assertEqual(conv('True'), True)
         self.assertEqual(conv('False'), False)