Feature Request: Dynamically Set Slack Channel

This issue is derived from: https://github.com/influxdata/kapacitor/issues/1404

Slack alert node currently supports setting channel in a given TICK script. However, it is currently not able to dynamically set the channel based on a tag or different field in an input measurement. It would definitely be a useful enhancement to have more flexibility for this channel to accept template inputs.

Start by tracing the Slack alert node and its existing TICK-script channel configuration. Determine how tag or field values could supply a template input, and consider the feature complete when a Slack channel can be selected dynamically from input data.

Written by the indexing model from the issue text.