Don't log a warning when slow_spawn_timeout is disabled

When using the `KubeSpawner` it is typical to disable the
`slow_spawn_timeout` by setting it to 0. `zero-to-jupyterhub-k8s`
does this by default [1]. However, this causes an immediate `TimeoutError`
which gets logged as a warning like this:

>User hub-stress-test-123 is slow to start (timeout=0)

This avoids the warning by checking the value and if disabled simply
returns without logging the warning.

[1] https://github.com/jupyterhub/zero-to-jupyterhub-k8s/commit/b4738edc5

Closes #3126

jupyterhub/handlers/base.py

@@ -977,6 +977,9 @@ class BaseHandler(RequestHandler):
             # waiting_for_response indicates server process has started,
             # but is yet to become responsive.
             if spawner._spawn_pending and not spawner._waiting_for_response:
+                # If slow_spawn_timeout is intentionally disabled then we
+                # don't need to log a warning, just return.
+                if self.slow_spawn_timeout > 0:
                     # still in Spawner.start, which is taking a long time
                     # we shouldn't poll while spawn is incomplete.
                     self.log.warning(