Wait until SSHABLE in volumes_extend test

Change-Id: Id6f4f7dd842cf7f578c37a64ee5c9f46c0a893f2
diff --git a/tempest/api/volume/test_volumes_extend.py b/tempest/api/volume/test_volumes_extend.py
index fcbc982..9066979 100644
--- a/tempest/api/volume/test_volumes_extend.py
+++ b/tempest/api/volume/test_volumes_extend.py
@@ -114,7 +114,7 @@
           if the action on the server fails.
         """
         # Create a test server. Will be automatically cleaned up on teardown.
-        server = self.create_server()
+        server = self.create_server(wait_until='SSHABLE')
         # Attach the volume to the server and wait for the volume status to be
         # "in-use".
         self.attach_volume(server['id'], volume['id'])