Fix the usage error for config

Can't use 'self.config' to read the tempest.conf, change to use 'CONF'

Change-Id: I911a35ce90a9b65aa7bf1341e9f927e95049e074
Closes-bug: 1284500
diff --git a/tempest/api/compute/test_live_block_migration_negative.py b/tempest/api/compute/test_live_block_migration_negative.py
index da0e4c4..e1a264e 100644
--- a/tempest/api/compute/test_live_block_migration_negative.py
+++ b/tempest/api/compute/test_live_block_migration_negative.py
@@ -38,7 +38,7 @@
     def _migrate_server_to(self, server_id, dest_host):
         _resp, body = self.admin_servers_client.live_migrate_server(
             server_id, dest_host,
-            self.config.compute_feature_enabled.
+            CONF.compute_feature_enabled.
             block_migration_for_live_migration)
         return body
 
diff --git a/tempest/api/compute/v3/test_live_block_migration_negative.py b/tempest/api/compute/v3/test_live_block_migration_negative.py
index 4d820d8..9e07478 100644
--- a/tempest/api/compute/v3/test_live_block_migration_negative.py
+++ b/tempest/api/compute/v3/test_live_block_migration_negative.py
@@ -39,7 +39,7 @@
     def _migrate_server_to(self, server_id, dest_host):
         _resp, body = self.admin_servers_client.live_migrate_server(
             server_id, dest_host,
-            self.config.compute_feature_enabled.
+            CONF.compute_feature_enabled.
             block_migration_for_live_migration)
         return body