add defaults for slackNotifier job DSL api

[ulich]

We are using the job dsl to describe our jobs. The slackNotifications method is deprecated in favor of the new slackNotifier method that comes from the extension points.

The old slackNotifications provided many defaults so we could configure e.g. the slackDomain, token, etc. globally in jenkins and then only add a small block like

    publishers {
        slackNotifications {
            projectChannel('channel')
            notifyUnstable()
            notifyFailure()
            notifyBackToNormal()
        }
    }

but with the new slackNotifier method we have to specify all the properties:

ERROR: the following options are required and must be specified: teamDomain, authToken, buildServerUrl, sendAs, startNotification, notifyAborted, notifyNotBuilt, notifySuccess, notifyRepeatedFailure, includeTestSummary, commitInfoChoice, includeCustomMessage, customMessage

Is it possible to add defaults with extension points?

[samrocketman]

Feel free to subscribe to #296 for updates related to Slack Plugin 2.2 release.