Fix a minor error in method _get_alternative_flavor

Current ServerDiskConfigTestJSON._get_alternative_flavor
defined in file test_disk_config.py cannot get the correct
alternative_flavor. The condition used to decide whether
the flavor_alt should be used can never return true because
a integer never equals to a string.

Change-Id: Ib4f06d0a93580d84fcea0f475695f2c9fd34856b
diff --git a/tempest/api/compute/servers/test_disk_config.py b/tempest/api/compute/servers/test_disk_config.py
index 5e9ee5c..0121c42 100644
--- a/tempest/api/compute/servers/test_disk_config.py
+++ b/tempest/api/compute/servers/test_disk_config.py
@@ -80,7 +80,7 @@
     def _get_alternative_flavor(self):
         resp, server = self.client.get_server(self.server_id)
 
-        if int(server['flavor']['id']) == self.flavor_ref:
+        if server['flavor']['id'] == self.flavor_ref:
             return self.flavor_ref_alt
         else:
             return self.flavor_ref