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
1 file changed