Fix request interval in _wait_for_lb_functional

The request_interval parameter is passed to the _wait_for_lb_functional
function but it wasn't used in the function.
Now the function sleeps for request_interval sec for each iteration of
the loop.
It mitigates some random failures with the TCP listener +
least_connection lb_algorithm tests that require a specific delay
between the requests [0]

[0] https://opendev.org/openstack/octavia-tempest-plugin/src/branch/master/octavia_tempest_plugin/tests/scenario/v2/test_traffic_ops.py#L854

Change-Id: Idf0a9fb71178abea732748f92f01eed5a55a3228
diff --git a/octavia_tempest_plugin/tests/validators.py b/octavia_tempest_plugin/tests/validators.py
index 5afe28a..34e8148 100644
--- a/octavia_tempest_plugin/tests/validators.py
+++ b/octavia_tempest_plugin/tests/validators.py
@@ -412,7 +412,7 @@
                     return
             except Exception:
                 LOG.warning('Server is not passing initial traffic. Waiting.')
-                time.sleep(1)
+            time.sleep(request_interval)
 
         LOG.debug('Loadbalancer wait for load balancer response totals: %s',
                   response_counts)