Update the rails secret generator code to smoothly add new secrets
A proposed fix for: #1326 (closed)
Replaces: !785 (merged)
Depends on: gitlab-org/build/CNG!226 (merged)
Have the secrets generator always do an apply for rails secrets But have it first populate the variables using any existing secret data it finds
This allows us to generate secrets for new keys added in new gitlab versions, while still respecting/retaining any existing key values.