Add template for mounting DB passwords from secrets

[tnederlof]

Adding a template to the rstudio-library chart, which will be used in future PRs to automatically mount PostgreSQL database passwords as environment variables from a secret.

This is to work on #422 (now that Workbench supports env variables) and #420.

Once this is in, will add to each product these values:

database:
  password:
    secretKey: "password"
    secret: ""