qa: assign prometheus ports during selftest

This was throwing IOError("Port 9283 not free on '::'",)
when trying to serve, since merging ceph#19744

It's because the standbys (on the same node as the active) are
now trying to listen too.

Fixes: https://tracker.ceph.com/issues/22755
Signed-off-by: John Spray <john.spray@redhat.com>

qa/tasks/mgr/test_module_selftest.py

@@ -31,6 +31,7 @@ def test_zabbix(self):
         self._selftest_plugin("zabbix")
 
     def test_prometheus(self):
+        self._assign_ports("prometheus", "server_port", min_port=8100)
         self._selftest_plugin("prometheus")
 
     def test_influx(self):