Story

As an admin of RHDH, I want to be able to configure mountPath and subPath of default mounted secrets so that Operator will handle it with no needs to configure it on deployment object. 

Background{}

In the deployment.yaml we have several optional mounted from secret volumes (like 
dynamic-plugins-npmrc, dynamic-plugins-registry-auth). This config can be moved to secret-files.yaml along with Secret declaration for clarity. Also, mounting secrets this way does not allow to have more than one Backstage instance per namespace as the volume name is hardcoded.
 
To make it work we also need to enable multi-object configuration (like in PVC) for secret-files.

Dependencies and Blockers

QE impacted work

Documentation impacted work

Acceptance Criteria

upstream documentation updates (design docs, release notes etc)

Technical enablement / Demo