alertmanager.installDefaultRules name may be confusing

[ralgozino]

In #159 we introduced the field .spec.distribution.modules.monitoring.alertmanager.installDefaultRules.

The installDefaultRules name of this field can be confusing to the user because, even when the field is false, Prometheus Rules are installed in the cluster, triggering alerts.

What this field disables is KFD default Alertmanager configuration for sending notifications for the aforementioned alerts.

Setting this field to true, let's the user create a custom Alertmanager configuration for routing the alerts.

I suggest renaming the field to something like installDefaultConfig instead.

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Another improvement regarding to this field is setting in the schema the related fields deadManSwitchWebhookUrl, and slackWebhookUrl mandatory when installDefaultRules is true.