feat(plugin-site-frontend): add PDB and unittest

[smerle33]

as per jenkins-infra/helpdesk#3719

plugin-site, plugin-site-frontend, PodDisruptionBudget (policy) has been added:
- 
+ # Source: plugin-site/templates/pdb-frontend.yaml
+ apiVersion: policy/v1
+ kind: PodDisruptionBudget
+ metadata:
+   name: plugin-site-frontend
+   labels:
+     app.kubernetes.io/name: plugin-site
+     app.kubernetes.io/instance: plugin-site
+     helm.sh/chart: plugin-site-0.3.0
+     app.kubernetes.io/version: "1.0"
+     app.kubernetes.io/managed-by: Helm
+     jenkins.io/maintainer: timja
+ spec:
+   minAvailable: 1
+   selector:
+     matchLabels:
+       app.kubernetes.io/name: plugin-site-frontend
+       app.kubernetes.io/instance: plugin-site