Avoid local variable referenced before assignment

The variable does not defined before it is referenced, this patch
add it.

Change-Id: Idf6436b99f1bd12aa6385f1d1bd293e212cf375f
diff --git a/tempest/api/compute/admin/test_flavors.py b/tempest/api/compute/admin/test_flavors.py
index 95e7ef1..fde5622 100644
--- a/tempest/api/compute/admin/test_flavors.py
+++ b/tempest/api/compute/admin/test_flavors.py
@@ -161,6 +161,7 @@
         verify_flavor_response_extension(flavor)
 
         # Check if flavor is present in list
+        flag = False
         flavors = self.user_client.list_flavors(detail=True)['flavors']
         for flavor in flavors:
             if flavor['name'] == flavor_name: