tests: make storcon scale test AZ-aware (#9952)

## Problem

We have a scale test for the storage controller which also acts as a
good stress test for scheduling stability. However, it created nodes
with no AZs set.

## Summary of changes

- Bump node count to 6 and set AZs on them.

This is a precursor to other AZ-related PRs, to make sure any new code
that's landed is getting scale tested in an AZ-aware environment.

test_runner/performance/test_storage_controller_scale.py

@@ -72,7 +72,7 @@ def test_storage_controller_many_tenants(
     we don't fall over for a thousand shards.
     """
 
-    neon_env_builder.num_pageservers = 5
+    neon_env_builder.num_pageservers = 6
     neon_env_builder.storage_controller_config = {
         # Default neon_local uses a small timeout: use a longer one to tolerate longer pageserver restarts.
         # TODO: tune this down as restarts get faster (https://github.com/neondatabase/neon/pull/7553), to
@@ -84,6 +84,11 @@ def test_storage_controller_many_tenants(
         compute_reconfigure_listener.control_plane_compute_hook_api
     )
 
+    AZS = ["alpha", "bravo", "charlie"]
+    neon_env_builder.pageserver_config_override = lambda ps_cfg: ps_cfg.update(
+        {"availability_zone": f"az-{AZS[ps_cfg['id'] % len(AZS)]}"}
+    )
+
     # A small sleep on each call into the notify hook, to simulate the latency of doing a database write
     compute_reconfigure_listener.register_on_notify(lambda body: time.sleep(0.01))