Increase number of retires for wait_for_traefik

Recent test runs have shown that the current number of retries is
not enough and traefik has still not updated its config. This PR
increases the number of retries in the hope that it will be
enough.

zaza/openstack/charm_tests/tempest/tests.py

@@ -186,7 +186,7 @@ def wait_for_dead_units(self, application_name, dead_count):
         assert len(dead_units) == dead_count
 
     @tenacity.retry(wait=tenacity.wait_exponential(multiplier=1, max=60),
-                    reraise=True, stop=tenacity.stop_after_attempt(8))
+                    reraise=True, stop=tenacity.stop_after_attempt(20))
     def wait_for_traefik(self, application_name):
         """Wait for traefk to finish processing lb changes."""
         logging.warning(