Slack consumer's message can't be configured

[chrisnovakovic]

Issue

It'd be nice to be able to configure the message that the Slack consumer sends to the announcement channel.

Expected behaviour

The message posted to Slack by the Slack consumer is customisable per pipeline.

Actual behaviour

The message posted to Slack by the Slack consumer is hardcoded, and follows the format

Dracon scan dracon-<SCANID> started on <DATE> has been completed with <N> issues