helm: add support for docker registry caches

[diegodelemos]

A docker registry cache has a few advantages for REANA:

• Less public Docker registry quota usage: Use organization's/intitute's docker registry cache to minimise the usage of public docker registry which might have limits, and reaching them can block the community (e.g. DockerHub limits already reached during rena-dev.cern.ch deployment).
• Hight Availability: survive in the events of the public docker registry being down. e.g. RS gets relocated from VM1 to VM2 because of an emergency. VM2 doesn't have reanahub/reana-server:0.8.0a1 so it needs to pull it. DockerHub is down, so REANA is down.
• Performance: REANA jobs often use large images. Having to pull those image from the internet is way slower than pulling them from the same computer center.

At CERN we have Harbor (registry.cern.ch) which is a CNCF Graduated Docker registry. The way it works is prefixing the docker image names with the registry cache server. If we add a new config to values.yaml e.g. container_registry_cache_server, then we can pretend this string to every image.

Note: Adding it to infrastructure components (RS, RWC) is easy. Adding it to runtime components is a bit more tedious and getting #277 done first would help.