add default role of 'prometheus_alerts' for alerts (#29111)

Currently, MIQ only supports an alert type of 'prometheus', so rather than have the caller of manageiq_provider pass this info, just set it as the default.

lib/ansible/modules/remote_management/manageiq/manageiq_provider.py

@@ -336,6 +336,7 @@ def supported_providers():
             authtype='bearer',
             default_role='default',
             metrics_role='prometheus',
+            alerts_role='prometheus_alerts',
         ),
         Amazon=dict(
             class_name='ManageIQ::Providers::Amazon::CloudManager',