Fix skip_checks in live_migration

use cls.skipException instead of skipTest
if there are less then 2 compute nodes

Change-Id: I1a3b5db3389777dd5e73e53419280918b9b97ed2
diff --git a/tempest/api/compute/admin/test_live_migration.py b/tempest/api/compute/admin/test_live_migration.py
index cfae772..7c4c30c 100644
--- a/tempest/api/compute/admin/test_live_migration.py
+++ b/tempest/api/compute/admin/test_live_migration.py
@@ -36,7 +36,7 @@
                         "not available" % cls.__name__)
             raise cls.skipException(skip_msg)
         if CONF.compute.min_compute_nodes < 2:
-            raise cls.skipTest(
+            raise cls.skipException(
                 "Less than 2 compute nodes, skipping migration test.")
 
     @classmethod