Upgrade tests: Make sure head node has enough memory to execute test

tests/upgrade/raycluster_sdk_upgrade_test.py

@@ -49,7 +49,7 @@ def run_mnist_raycluster_sdk_oauth(self):
                 namespace=self.namespace,
                 num_workers=1,
                 head_cpus=1,
-                head_memory=2,
+                head_memory=4,
                 worker_cpu_requests=1,
                 worker_cpu_limits=1,
                 worker_memory_requests=4,