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).

Confronta il Jenkinsfile di questo repository con il Jenkinsfile di controller-sdk-go, prestando particolare attenzione ai passaggi duplicati introdotti da controller-sdk-go#46. Leggi la documentazione di Pipeline Global Library e workflow-cli#133 per l’esempio di notifica via e-mail; il lavoro è completato quando la logica condivisa è centralizzata ed entrambi i pipeline la utilizzano.

Scritto dal modello di indicizzazione a partire dal testo della issue.