Allow deleting a namespace from a groovy pipeline

In the context of testing upgrading from a nuxeo version to another, we need to deploy both versions sequentially without nesting them.

To do so we need to call methods granularly, to deploy, redeploy and then delete without having too much automation and logging around it.

We can actually deploy and destroy a helmfile, but the namespace will remain.
This forces us to delete the namespace manually after the test.

Exposing the existing deleteNamespace method would ensure it is always done the same way.