make k8sgitlab secret configurable

this MR implements the option to use another gitlab secret for the deploy key. The secret needs to be created outside of the helm chart

ping @fabian.kirchner

merged

assigned to @Chilipp

enabled an automatic merge when the pipeline for 6b4ca616 succeeds

mentioned in commit f639506e

mentioned in commit hcdc/kubernetes/charts/nginx@99035e42

mentioned in merge request hcdc/kubernetes/charts/nginx!19 (merged)