Using `replace-literal` to insert secrets leaks the secrets through
the `replace-literal` process' `/proc/<pid>/cmdline`
file. `replace-secret` solves this by reading the secret straight from
the file instead, which also simplifies the code a bit.
This reverts commit d9e18f4e7f77fffde95384d36cc8ac5d1d51b356.
This change is broken, since it doesn't configure the proper database
username in keycloak when provisioning a local database with a custom
username. Its intended behavior is also potentially confusing and
dangerous, so rather than fixing it, let's revert to the old one.
