fix(monitoring): fire IpmiCollectorDown after 15m

IPMI is not the most reliable of things and it does
not make sense to fire it often, let's wait 15 minutes
before we make a lot of noise.
diff --git a/roles/kube_prometheus_stack/files/jsonnet/legacy.libsonnet b/roles/kube_prometheus_stack/files/jsonnet/legacy.libsonnet
index 50ea305..b11d8a6 100644
--- a/roles/kube_prometheus_stack/files/jsonnet/legacy.libsonnet
+++ b/roles/kube_prometheus_stack/files/jsonnet/legacy.libsonnet
@@ -7,6 +7,7 @@
           rules: [
             {
               alert: 'IpmiCollectorDown',
+              'for': '15m',
               expr: 'ipmi_up == 0',
               labels: {
                 severity: 'warning',