Support existing secrets/config mapping

[bufke]

Use case: I'd like to store a non-helm managed secret or configmap and map it to kpi env vars. This is nice for shared credentials, such as sharing AWS keys between multiple staging servers

[bufke]

This might be a good task @LMNTL but consider it low priority. We should follow bitnami standards for this. See the "existingSecret" value for example. The use case is this:

I have 0-to-infinite staging servers in a particular namespace. Perhaps they are feature branch review instances. Right now, I need to maintain a lot of yaml files to set each one up. I could reduce a lot of this copied configuration if I instead referenced a shared secret or config map. Maybe all my staging servers use the same AWS credentials for django-storages. Instead of copying yaml, I can create 1 configmap and 1 secret outside of helm. Then point all my helm instances to this one config/secret. Right now - I think this would only reduce the yaml. Maybe some day it would eliminate it entirely and a --reuse-values could be applied on a very limited per-instance configuration.