Raise AssertionError instead of eventlet.timeout.Timeout when failing.

The eventlet.timeout.Timeout exception subclasses Python's BaseException.
This is problematic because that is a higher category exception than
Python's Exception. As a result of this type of exception being raised
during testing, this halts the normal test flow.
AssertionError will properly mark a test as failed.

Change-Id: Iaf74f877f6a70c0383d539e9b01d6db2ec0ddeca
Closes-Bug: 1654283
diff --git a/neutron/tests/tempest/api/test_routers.py b/neutron/tests/tempest/api/test_routers.py
index 20e54cf..d98ea3a 100644
--- a/neutron/tests/tempest/api/test_routers.py
+++ b/neutron/tests/tempest/api/test_routers.py
@@ -163,7 +163,7 @@
         intf = self.create_router_interface(router['id'], subnet['id'])
         status_active = lambda: self.client.show_port(
             intf['port_id'])['port']['status'] == 'ACTIVE'
-        utils.wait_until_true(status_active)
+        utils.wait_until_true(status_active, exception=AssertionError)
 
     @test.idempotent_id('c86ac3a8-50bd-4b00-a6b8-62af84a0765c')
     @test.requires_ext(extension='extraroute', service='network')