Extract common Jenkinsfile logic into Pipeline Global Library

This repo's [Jenkinsfile](https://github.com/deis/workflow-cli/blob/master/Jenkinsfile) as well as the [controller-sdk-go](https://github.com/deis/controller-sdk-go) repo's [Jenkinsfile](https://github.com/deis/controller-sdk-go/blob/master/Jenkinsfile) share common logic, especially as of https://github.com/deis/controller-sdk-go/pull/46 which builds workflow-cli in place, duplicating many steps used in this repo's pipeline.

It would be preferable if we extracted shared logic into common functionality via the [Pipeline Global Library](https://github.com/jenkinsci/workflow-cps-global-lib-plugin/blob/master/README.md#pipeline-global-library) to both reduce duplication and enable only making changes to such functionality in one location. Some functionality will even prove to be generally useful outside of workflow-cli/controller-sdk-go's scope, such as [sending email notifications on e2e failures](https://github.com/deis/workflow-cli/pull/133).

Vergleiche das Jenkinsfile dieses Repos mit dem Jenkinsfile von controller-sdk-go und achte besonders auf die durch controller-sdk-go#46 eingeführten duplizierten Schritte. Lies die Dokumentation zu Pipeline Global Library und workflow-cli#133 für das Beispiel zur E-Mail-Benachrichtigung; erledigt ist die Aufgabe, wenn die gemeinsame Logik zentralisiert ist und beide Pipelines sie verwenden.

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