Fix TypeError exception in test_server_actions setup

test_server_actions use rebuild_server from base class which returns
server's id. But this tests try to fetch 'server' from return value
which results to TypeError.

Change-Id: I5bbfbe6ee4e8acc84112d134d331380a69b637d0
Closes-Bug: #1514685
diff --git a/tempest/api/compute/servers/test_server_actions.py b/tempest/api/compute/servers/test_server_actions.py
index a59cb16..0754d27 100644
--- a/tempest/api/compute/servers/test_server_actions.py
+++ b/tempest/api/compute/servers/test_server_actions.py
@@ -52,7 +52,7 @@
         except Exception:
             # Rebuild server if something happened to it during a test
             self.__class__.server_id = self.rebuild_server(
-                self.server_id, validatable=True)['server']
+                self.server_id, validatable=True)
 
     def tearDown(self):
         self.server_check_teardown()