Support to set severity while sending alerts to Alerta

Alerta supports lot of severity levels. Below is a link for the full list.
http://docs.alerta.io/en/latest/api/alert.html#severity-table

As of today there is a hard mapping of Kapacitor's AertNode critical, warning and Info levels to on of the Alerta's severities.
There is no possibility of setting severity of alerts when sending to Alerta apart from what kapacitor already mapped.

This issue is being raised to request the support for setting the severity for alerts that are being sent to Alerta from Kapacitor.

Start by reviewing Kapacitor's Alerta alert sender and its existing mapping from critical, warning, and info levels, then compare the requested options with Alerta's severity table. Done means alerts sent to Alerta can use a caller-configured severity rather than only the current hard mapping, with coverage for the supported behavior.

Written by the indexing model from the issue text.