jenkins: Run Jenkins in Kubernetes

Running Jenkins in Kubernetes is relatively straightforward.  The
Kubernetes plugin automatically discovers all the connection and
authentication configuration, so a `kubeconfig` file is no longer
necessary.  I did set the *Jenkins tunnel* option, though, so that
agents will connect directly to the Jenkins JNLP port instead of going
through the ingress controller.

Jobs now run in pods in the *jenkins-job* namespace instead of the
*jenkins* namespace.  The latter is now where the Jenkins controller
runs, and the controller should not have permission to modify its own
resources.

jenkins/kustomization.yaml

@@ -0,0 +1,12 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+- jenkins.yaml
+
+secretGenerator:
+- name: imagepull-gitea
+  namespace: jenkins
+  type: kubernetes.io/dockerconfigjson
+  files:
+  - .dockerconfigjson