According to MongoDB documentation[1], "enabling internal authentication
(in a replicaset) enables access control". Therefore, if MongoDB is
configured without authentication (`auth = no`) a `keyFile` line
must not appear in the configuration file.
This commit attempts to solve the problem described above. An extra
check has been added to ensure the `keyFile` configuration option is
added only if replicaset authorization _and_ authentication is enabled.
[1]: https://docs.mongodb.org/v3.0/tutorial/enable-internal-authentication/