Assert that the PostgreSQL version being deployed is the one used
upstream. Allow the user to override this assertion, since it's not
always possible or preferable to use the recommended one.
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.
Bash doesn't handle subshell errors properly if the result is used as
input to a command. To cause the services to fail when the files can't
be read, we need to assign the value to a variable, then export it
separately.
