Fix an issue when nodes are not available in standalone test

When there is no node available, _try_to_associate_instance raises
which bypassed waiting.

Noticed here: https://1b623bcb8c866ffba3bc-dbe87819ad7b6716d83fae44185f9634.ssl.cf1.rackcdn.com/682029/1/gate/ironic-standalone/fc69fe9/job-output.txt

Change-Id: I7bcd804d3e01d80f977d428e6a084f4b7c7e1932
diff --git a/ironic_tempest_plugin/tests/scenario/baremetal_standalone_manager.py b/ironic_tempest_plugin/tests/scenario/baremetal_standalone_manager.py
index bbd7782..98e335d 100644
--- a/ironic_tempest_plugin/tests/scenario/baremetal_standalone_manager.py
+++ b/ironic_tempest_plugin/tests/scenario/baremetal_standalone_manager.py
@@ -205,6 +205,8 @@
 
         def _try_to_associate_instance():
             n = node or cls.get_random_available_node()
+            if n is None:
+                return False
             try:
                 cls._associate_instance_with_node(n['uuid'], instance_uuid)
                 nodes.append(n)