Add slack plugin support

[apetres]

Add support for Slack plugin

[daspilker]

Syntax for the Slack plugin is already provided by the Automatically Generated DSL.

[apetres]

That is true, the automatically generated DSL works as expected for this plugin. Unfortunately it does not work with RestApiScriptRunner (a useful tool which allows updating jobs via Jenkins's REST API). That is the reason I want to add explicit support.

[daspilker]

@apetres Maintaining the build-in DSL is a tedious task. Plugin maintainers are changing the config XML (although maintaining compatibility is not hard) and thus making Job DSL incompatible to newer plugin versions. And maintaining Job DSL for multiple versions of a plugin is even more tedious. E.g. support for the Slack plugin has initially been added in Job DSL 1.36 (August 2015). In May 2016 with release 2.0 of the Slack plugin it became incompatible and bug reports for Job DSL popped up because the generated config was incompatible. Instead of trying to keep up with changes of 1000+ plugins, I'm deprecating and eventually removing support for incompatible plugins if support for automatically generated DSL is available for that plugin. So DSL support for the Slack plugin has been deprecated in version 1.47 (May 2016) and removed with version 1.59 (March 2017) after the automatically generated DSL was available.

I'm aware that the automatically generated DSL is not available in every environment. But the configure block can be used as a fallback or the (with a lot more effort) the Jenkins Test Harness can be integrated to emulate a full Jenkins environment.