Fix pass incorrect volume_size while building bdm

If specify device_name, snapshot_id, delete_on_termination=True
and not specify volume_size for bdm while create a server,
heat will pass the bdm such as {'vdd': '1234:snap:0:True'} to
novaclient, the volume_size is incorrect,
nova won't create a volume from the snapshot because the
volume_size=0. If not specify volume_size, just to pass ''.

Change-Id: Ic1407671cc7bc6695bcf8e0c9f86fbdbaf245658
Closes-Bug: #1335728

heat/engine/resources/server.py

@@ -572,8 +572,10 @@ class Server(stack_user.StackUser):
 
             volume_size = mapping.get(cls.BLOCK_DEVICE_MAPPING_VOLUME_SIZE)
             delete = mapping.get(cls.BLOCK_DEVICE_MAPPING_DELETE_ON_TERM)
-            if volume_size or delete:
-                mapping_parts.append(str(volume_size or 0))
+            if volume_size:
+                mapping_parts.append(str(volume_size))
+            else:
+                mapping_parts.append('')
             if delete:
                 mapping_parts.append(str(delete))
 

heat/tests/test_server.py

@@ -1678,8 +1678,8 @@ class ServersTest(HeatTestCase):
         self.assertIsNone(servers.Server._build_block_device_mapping(None))
 
         self.assertEqual({
-            'vda': '1234:',
-            'vdb': '1234:snap',
+            'vda': '1234::',
+            'vdb': '1234:snap:',
         }, servers.Server._build_block_device_mapping([
             {'device_name': 'vda', 'volume_id': '1234'},
             {'device_name': 'vdb', 'snapshot_id': '1234'},
@@ -1687,7 +1687,7 @@ class ServersTest(HeatTestCase):
 
         self.assertEqual({
             'vdc': '1234::10',
-            'vdd': '1234:snap:0:True'
+            'vdd': '1234:snap::True'
         }, servers.Server._build_block_device_mapping([
             {
                 'device_name': 'vdc',