re-apply alert logic when dashboard configuration changes

[costin]

Ideally, the existing alerts that are affected by certain actions, should be discarded and the logic re-applied since the stale info can be misleading.
For example, opening the dashboard against a missing index gives an alert but after changing the index, the alert is still present so the user (me) assumes the index was introduced wrongly (which is not the case).

[spalger]

With Kibana 4 this shouldn't be an issue. Closing for now.