Optionally allow passing in an existing volume for the configuration template

[my0n]

Currently, the configuration template can only be passed in as a string via the parameter configuration.template. This is great for me (I use ArgoCD so having the whole config in git, and under one Application resource, is awesome), but there's probably a few other ways users would like to use the helm chart:

• pass in an existing ConfigMap or Secret as a template with fields that will be populated by the envsubst initContainer
• pass in an existing ConfigMap or Secret without using the template feature
• use a file on an existing volume somewhere that can be manually/programmatically modified
• use that file, but without templating