Add priority classname

This MR is the continuation of the !239 (closed)

The contributor requests that we continue the work started in his MR

The MR seems already reviewed and wasn't merged because of conflicts

• Manual test
• LGTM 🚀
• Conflict on merge

What does this MR do?

Add priorityClass support

Why was this MR needed?

What's the best way to test this MR?

$ kubectl create priorityclass high-priority --value=1000000 --description="high priority"
$ helm install . --generate-name
$ kubectl get pod chart-1596534669-gitlab-runner-769c5c86d-fwdw8 -o yaml | grep priority
    priority: 1000000
    priorityClassName: high-priority

What are the relevant issue numbers?

close #176 (closed)