[alerta] Support the "correlate" attribute

See http://alerta.readthedocs.io/en/latest/server.html#simple-correlation. Add support for the `correlate` attribute so when an alert with the same `resource` is received with an event in the
correlate list of related events with any severity, then the alert is correlated.

Start with the Alerta simple-correlation documentation linked in the issue and trace Kapacitor's alert-receiving and correlation handling entry points. Done means an alert with the same resource is correlated when its event appears in the correlate list, regardless of the related event's severity, with coverage for this behavior.

Written by the indexing model from the issue text.